Review on recent progress in chitosan-based hydrogels for wastewater treatment application

Rapid preparation of PAM/N-CNT nanocomposite hydrogels by DEM frontal polymerization and its performance study

In this study, a simple and eco-friendly method was proposed to efficiently prepare nanocomposite hydrogels with excellent mechanical properties and satisfactory pH response behaviour by frontal polymerization (FP) of DEM in close to 4 minutes. Acrylamide (AM) and choline chloride (ChCl) were used as raw materials to synthesize deep eutectic monomers (DEMs). Nitrogen-doped carbon nanotubes were dispersed in DEMs as fillers, and poly(acrylamide)/nitrogen-doped carbon nanotube (PAM/N-CNT) nanocomposite hydrogels were prepared by FP. The non-covalent interactions between PAM hydrogels and N-CNTs was verified by Fourier infrared spectroscopy. The mechanical properties of PAM/N-CNT nanocomposite hydrogels were investigated, as well as the swelling and pH response properties. The results showed that the compressive strength of PAM hydrogels was significantly enhanced by the addition of N-CNTs due to the hydrophobic interaction of N-CNTs, which also causes sensitive response properties of the PAM hydrogels in acid solution.

In this study, a simple and eco-friendly method was proposed to efficiently prepare nanocomposite hydrogels with excellent mechanical properties and satisfactory pH response behaviour by frontal polymerization of DEM in close to 4 minutes.

Chitosan as additive affects the bacterial community, accelerates the removals of antibiotics and related resistance genes during chicken manure composting

Manures, storages for antibiotic resistance genes (ARGs), pollute soil and water as well as endanger human health. Recently, we have been searching a better solution to remove antibiotics and ARGs during aerobic composting. Here, the dynamics of chitosan addition on the profiles of 71 ARGs, bacterial communities, chlortetracycline (CTC), ofloxacin (OFX) were investigated in chicken manure composting and compared with zeolite addition. Chitosan addition effectively reduces antibiotics contents (CTC under detection limit, OFX 90.96%), amounts (18) and abundance (56.7%, 11.1% higher than zeolite addition) of ARGs and mobile genetic elements (MGEs) after 42 days composting. Network analysis indicated that a total of 27 genera strains assigned into 4 phyla (Firmicutes, Proteobacteria, Actinobacteria and Bacteroidetes) were the potential hosts of ARGs. Redundancy analysis (RDA) demonstrated that bacterial community succession is the main contributor in the variation of ARGs. Overall, chitosan addition may effect bacterial composition by influencing physic-chemical properties and the concentration of antibiotics, Cu²⁺, Zn²⁺ to reduce the risk of ARG transmission. This study gives a new strategy about antibiotics and ARGs removal from composting on the basis of previous studies.

Novel amidinothiourea-modified chitosan microparticles for selective removal of Hg(II) in solution

Glutaraldehyde-crosslinked chitosan microparticles (CGP) prepared via the inversed-phase emulsification were successively modified by epichlorohydrin (ECH) and amidinothiourea (AT) as novel adsorbent (CGPET) for selective removal of Hg(II) in solution. FTIR, EA, XPS, SEM-EDX, TG, DTG, and XRD results indicated that CGPET had ample -NH₂ and CS, relative rough surface, mean diameter of ~40 μm, great thermal stability, and crystalline degree of 2.4%, beneficial to the uptake of Hg(II). The optimum parameters (pH 5, dosage 1 g/L, contact time 4 h, and initial concentration 150 mg/L) were acquired via batches of adsorption experiments. Adsorption behavior was well described by the Liu isothermal and pseudo-second-order kinetics models, and the maximum adsorption capacity was 322.51 mg/g, surpassing many reported adsorbents. Regeneration and coexisting-ion tests demonstrated that CGPET had outstanding reusability (R_r > 86.89% at the fifth cycle) and selectivity (R_s > 93%). Besides, its potential adsorption sites and mechanisms were proposed.

Mesoporous cellulose-chitosan composite hydrogel fabricated via the co-dissolution-regeneration process as biosorbent of heavy metals

Developing low-cost and high-performance biosorbent for water purification continues drawing more and more attention. In this study, cellulose-chitosan composite hydrogels were fabricated via a co-dissolution and regeneration process using a molten salt hydrate (a 60 wt% aqueous solution of LiBr) as a solvent. The addition of chitosan not only introduced functionality for metal adsorption but also increased the specific surface area and improved the mechanical strength of the composite hydrogel, compared to pure cellulose hydrogel. Batch adsorption experiments indicated that the composite hydrogel with 37% cellulose and 63% chitosan exhibited an adsorption capacity of 94.3 mg/g (1.49 mmol/g) toward Cu²⁺ at 23 °C, pH 5, and initial metal concentration of 1500 mg/L, which was 10 times greater than the adsorption capacity of pure cellulose hydrogel. Competitive adsorption from a mixed metals solution revealed that the cellulose-chitosan composite hydrogel exhibited selective adsorption of the metals in the order of Cu²⁺ > Zn²⁺ > Co²⁺. This study successfully demonstrated an innovative method to fabricate biosorbents from abundant and renewable natural polymers (cellulose and chitosan) for removing metal ions from water.

Preparation and Characterization of Chitosan/Bentonite Composites for Cr (VI) Removal from Aqueous Solutions

Chitosan/bentonite composites (CSBT) prepared by physical gelation were tested for the adsorption of Cr (VI) from aqueous solutions in this work. The composites were prepared at a mass ratio from 2 : 1 to 1 : 2, and a composite of 1 : 1 was found to be most suitable for efficient Cr (VI) removal. The influencing parameters, including temperature, adsorbent dose, and pH, were statistically optimized using response surface methodology (RSM) for the removal of Cr (VI). The pH was found to be the limiting factor during the adsorption process, and under the optimal conditions, namely, adsorbent dose of 400 mg/L, $\text{pH}=3$ , and temperature of 298 K, 87.61% Cr (VI) would be removed expectantly. The mechanism of Cr (VI) removal by CSBT was discussed, and the protonation of amino groups on chitosan followed by the combination of -NH3+ and anionic hexavalent chromium was the primary driving force. In addition, the removal of Cr (VI) onto CSBT was monolayer adsorption with a maximum adsorption capacity of 133.85 mg/g by the Langmuir isotherm. CSBT follows a pseudosecond-order kinetic model, and within 1.5 h, adsorption was observed to reach equilibrium. The calculated thermodynamic functions clarified that the adsorption process was exothermic and spontaneous below 312.60 K. CSBT could be regenerated after desorption by 0.5 mol/L NaOH solutions and exhibited superior reusability after six cycles. This study demonstrated composites of chitosan/bentonite as eco-friendly bioadsorbents for the removal of Cr (VI) from aqueous environments.

Assessment of Casuarina glauca as biofiltration model of secondary treated urban wastewater: effect on growth performances and heavy metals tolerance

The use of fast-growing tree species, such as Casuarina glauca for wastewater treatment could improve the quality of wastewater and offer an ecological and sustainable system. A hydroponically experiment was conducted to evaluate C. glauca ability to remove heavy metals from secondary treated urban wastewater (SWW). The effect of the SWW on plant biomass, some physiological parameters, heavy metals (Cd, Pb, Ni and Zn) bioaccumulation and removal from wastewater was evaluated. After 28 days, wastewater treatment C. glauca showed high efficiency for the removal of pathogenic bacteria such as faecal coliforms and faecal streptococci from SWW. A significant reduction was found for electrical conductivity, biochemical oxygen demand, chemical oxygen demand and suspended solids with 31%, 92%, 83% and 31% respectively. Casuarina glauca plants were able to remove heavy metal ions Cd, Pb, Ni and Zn from SWW and the removal efficiency was 92%, 77%, 83% and 73%, respectively. Casuarina glauca plants accumulated concentrations of heavy metals (Cd, Pb, Ni and Zn) in their roots higher than the shoots. SWW had a remarkable effect on plant growth and photosynthetic capacity in C. glauca compared with plants grown in tap water (control). The results indicated that C. glauca can act as scavengers of heavy metal ions from polluted water and confirms their ability for wastewater treatment.

Chitosan hydrogel synthesis to remove arsenic and fluoride ions from groundwater

Groundwater samples from eight deep drinking water wells that cover three aquifers in Chihuahua City, northern Mexico, were fully characterized. Water is naturally contaminated with arsenic, fluoride, and uranium, according to the Environmental Protection Agency (EPA) and local standards. The results from the Geochemist's Workbench (GWB) program revealed that the minerals in equilibrium with the groundwater were calcite and dolomite, while others, such as fluoride, schoepite, rutherfordite and K(UO₂)(AsO₄), were also dissolved. The hydrogeochemical characterization of water samples indicates that they were sodium bicarbonate-type water samples at neutral to slightly alkaline pH (7.6-8.3). A batch equilibrium sorption procedure was implemented using natural groundwater, a synthesized chitosan network (net-CS) and a chitosan binary network grafted with N-vinylcaprolactam/N-N-dimethylacrylamide (net-CS)-g-NVCL/DMAAm hydrogels. Isotherms and kinetics sorption tests were evaluated. The adsorption capacity of net-CS hydrogels for As ions was 0.0022 mg/g and F ions 0.15 mg/g after 50 h. Scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS) was used to investigate the hydrogel surface before and after the sorption process, and TGA was used to evaluate the stability of the adsorbents. Freundlich adsorption isotherm constants for As and F ions indicate heterogeneous sorption and the mechanism of retention by physisorption.

Role of chitosan-based hydrogels in pollutants adsorption and freshwater harvesting: A critical review

The shortage of freshwater resources is an urgent problem worldwide, especially for some areas that lack rainfall conditions. The development of reliable wastewater treatment and freshwater harvesting equipment has become an urgent demand. Hydrogel is a porous 3D network structure with good pollutant adsorption capacity, water holding capacity, water adsorption capacity, and reversible swelling ability, which has been widely used in water treatment. Chitosan (CH), as the abundant bioactive material in nature, is commonly used to prepare hydrogels with low-cost, favorable stability, good antimicrobial activity, high mechanical properties, biodegradability, and environmental friendliness. Therefore, this review presents a comprehensive review of the various applications of CH-based hydrogels in water treatment including various pollutant adsorption, oil-water separation, seawater desalination, and atmospheric condensation. The relevant mechanisms, application potential, and challenge are also illustrated. This review aims to provide a viable idea to address the shortage of freshwater resources.

Extended Release of Metronidazole Drug Using Chitosan/Graphene Oxide Bionanocomposite Beads as the Drug Carrier

This study depicts the facile approach for the synthesis of chitosan/graphene oxide bionanocomposite (Chi/GO) beads via the gelation process. This is the first-ever study in which these Chi/GO beads have been utilized as a drug carrier for the oral drug delivery of metronidazole (MTD) drug, and investigations were made regarding the release pattern of the MTD drug using these Chi/GO beads as a drug carrier for a prolonged period of 84 h. The MTD is loaded on the surface as well as the cavity of the Chi/GO beads to result in MTD-Chi/GO bionanocomposite beads. The MTD drug loading was found to be 683 mg/g. Furthermore, the in vitro release patterns of pure drug and the drug encapsulated with Chi/GO beads are explored in simulated gastric as well as simulated intestinal fluids with phosphate-buffered saline (PBS) of pH 1.2 and 7.4, respectively. As-synthesized bionanocomposite beads have shown excellent stability and capacity for extended release of the MTD drug as compared to the pure drug in terms of bioavailability in both media. The cumulative release data are fitted with the Korsmeyer-Peppas kinetics and first-order reaction kinetics at pH 1.2 and 7.4. The synthesized bionanocomposite beads have good potential to minimize the multiple-dose frequency with the sustained drug release property and can reduce the side effects due to the drug.

Facile preparation of taurine modified magnetic chitosan nanocomposites as biodegradable adsorbents toward methylene blue

A novel magnetic Fe₃O₄@chitosan@taurine adsorbent (MCT) was prepared by surface modification of magnetic chitosan nano-composites with taurine-glutaraldehyde solution. The adsorbents were characterized by FTIR, SEM, TEM, XRD, TGA and VSM techniques, respectively. According to the FTIR spectrum of MCT, the characteristic peaks of the SO₃^-group on MCT were observed to have a shift after adsorption of the cationic dye, which indicates that there may be electrostatic attraction between the MCT and the cationic dye. Moreover, the saturation magnetization of MCT was found to be 20.797 emu g^-1, suggesting that MCT has sufficient magnetic response to meet the need of magnetic separation. The adsorption properties of cationic dyes by MCT were further investigated by using methylene blue (MB) as a representative. The adsorption behaviour of MB by MCT was well described by the pseudo-second order kinetic model and the Langmuir isotherm model, respectively. The maximum adsorption capacity of MB calculated from the Langmuir model fitting was 204.1 mg g^-1 at pH 5 and 384.6 mg g^-1 at pH 9, respectively, and the adsorption equilibrium could be reached within 10 min. Besides, the negative values of ΔG° and ΔH° suggested that the adsorption process was spontaneous and exothermic, and the good reusability indicated that MCT could act as a recyclable adsorbent for dye adsorption. All of these results illustrate that MCT has great potential for practical application in removal of cationic dyes from aqueous solutions.

Tough chitosan/poly(acrylamide-acrylic acid)/cellulose nanofibrils/ethylene glycol nanocomposite organohydrogel with tolerance to hot and cold environments

Simultaneously achieving good mechanical properties and high tolerance to hot and cold environments in hydrogel materials remains a challenge. In this work, ethylene glycol (EG) and cellulose nanofibrils (CNFs) were introduced into chitosan/poly(acrylamide-acrylic acid) double-network hydrogels to improve their toughness and tolerance to hot and cold environments. The effect of EG and CNFs on the properties of the hydrogels was studied respectively. EG increases the tolerance of the hydrogel to hot and cold environments. However, EG had a negative effect on the mechanical properties of hydrogels. In addition, CNFs substantially enhanced the strength and toughness of the chitosan/poly(acrylamide-acrylic acid)/EG organohydrogels. Finally, with the cooperative action of EG and CNFs, high-strength and tough organohydrogels (tensile strength = 0.71 MPa, elongation at break = 787.2%) with a high tolerance to hot and cold environments (-23 °C to 60 °C) were obtained. Further, EG enabled the organohydrogel to revert to its original state after drying at 60 °C. This paper provides a new route to prepare high-strength and tough organohydrogels with a high tolerance to hot and cold environments.

Graphene-Based Materials Immobilized within Chitosan: Applications as Adsorbents for the Removal of Aquatic Pollutants

Graphene and its derivatives, especially graphene oxide (GO), are attracting considerable interest in the fabrication of new adsorbents that have the potential to remove various pollutants that have escaped into the aquatic environment. Herein, the development of GO/chitosan (GO/CS) composites as adsorbent materials is described and reviewed. This combination is interesting as the addition of graphene to chitosan enhances its mechanical properties, while the chitosan hydrogel serves as an immobilization matrix for graphene. Following a brief description of both graphene and chitosan as independent adsorbent materials, the emerging GO/CS composites are introduced. The additional materials that have been added to the GO/CS composites, including magnetic iron oxides, chelating agents, cyclodextrins, additional adsorbents and polymeric blends, are then described and discussed. The performance of these materials in the removal of heavy metal ions, dyes and other organic molecules are discussed followed by the introduction of strategies employed in the regeneration of the GO/CS adsorbents. It is clear that, while some challenges exist, including cost, regeneration and selectivity in the adsorption process, the GO/CS composites are emerging as promising adsorbent materials.

A comprehensive survey upon diverse and prolific applications of chitosan-based catalytic systems in one-pot multi-component synthesis of heterocyclic rings

Heterocyclic compounds are among the most prestigious and valuable chemical molecules with diverse and magnificent applications in various sciences. Due to the remarkable and numerous properties of the heterocyclic frameworks, the development of efficient and convenient synthetic methods for the preparation of such outstanding compounds is of great importance. Undoubtedly, catalysis has a conspicuous role in modern chemical synthesis and green chemistry. Therefore, when designing a chemical reaction, choosing and or preparing powerful and environmentally benign simple catalysts or complicated catalytic systems for an acceleration of the chemical reaction is a pivotal part of work for synthetic chemists. Chitosan, as a biocompatible and biodegradable pseudo-natural polysaccharide is one of the excellent choices for the preparation of suitable catalytic systems due to its unique properties. In this review paper, every effort has been made to cover all research articles in the field of one-pot synthesis of heterocyclic frameworks in the presence of chitosan-based catalytic systems, which were published roughly by the first quarter of 2020. It is hoped that this review paper can be a little help to synthetic scientists, methodologists, and catalyst designers, both on the laboratory and industrial scales.

Influence of selenium precursors on the formation of iron selenide nanostructures (FeSe2): Efficient Electro-Fenton catalysts for detoxification of harmful organic dyestuffs

In this investigation, a sequences of iron diselenide (FeSe₂) nanomaterials as the competent and highly stable catalysts for the detoxification of aqueous organic dye pollutants such as Congo red (CR) and methylene blue (MB) through Electro-Fenton (EF) process using hydrogen peroxide as an initiator have been studied. The utilized selenium precursors include selenium metal, selenious acid (H₂SeO₃) and selenium dioxide (SeO₂) which were employed for the synthesis of FeSe₂ catalysts through a wet chemical strategy. It has been observed that based on the employed precursors, different morphologies ranges of the FeSe₂ catalysts were obtained: microgranualr particles to nano-stick to nanoflakes. The crystalline nature and phase purity of the obtained FeSe₂ catalysts were determined through XRD, Raman and HR-TEM analyses which confirmed their orthorhombic ferroselite structure. Among the prepared FeSe₂ catalysts, FS-2 (using H₂SeO₃) displayed better porous properties as compared to other catalysts and achieved the highest surface area of 74.68 m²g^-1. The narrow bandgap (0.88 eV) and fast conversion of Fe²⁺/Fe³⁺ cycle of FeSe₂ led CR and MB degradation of 93.3% and 90.4%, respectively. The outcome of this study demonstrates improved catalytic properties of FeSe₂ nanostructures for the efficient detoxification of hazardous and toxic effluents.

Chitosan-based blends for biomedical applications

Polysaccharides are the most abundant naturally available carbohydrate polymers; composed of monosaccharide units covalently connected together. Chitosan is the most widely used polysaccharides because of its exceptional biocompatibility, mucoadhesion, and chemical versatility. However, it suffers from a few drawbacks, e.g. poor mechanical properties and antibacterial activity for biomedical applications. Blending chitosan with natural or synthetic polymers may not merely improve its physicochemical and mechanical properties, but may also improve its bioactivity-induced properties. This review paper summarizes progress in chitosan blends with biodegradable polymers and polysaccharides and their biomedical applications. Blends of chitosan with alginate, starch, cellulose, pectin and dextran and their applications were particularly addressed. The critical and challenging aspects as well as the future ahead of the use of chitosan-based blends were eventually enlightened.

Characterization of Different Salt Forms of Chitooligosaccharides and Their Effects on Nitric Oxide Secretion by Macrophages

In this paper, chitooligosaccharides in different salt forms, such as chitooligosaccharide lactate, citrate, adipate, etc., were prepared by the microwave method. They were characterized by SEM, FTIR, NMR, etc., and the nitric oxide (NO) expression was determined in RAW 264.7 cells. The results showed that pure chitooligosaccharide was an irregular spherical shape with rough surface, and its different salt type products are amorphous solid with different honeycomb sizes. In addition to the characteristic absorption peaks of chitooligosaccharides, in FTIR, the characteristic absorption of carboxyl group, methylene group, and aromatic group in corresponding acid appeared. The characteristic absorption peaks of carbon in carboxyl group, hydrogen and carbon in methyl, methylene group, and aromatic group in corresponding acid also appeared in NMR. Therefore, the sugar ring structure and linking mode of chitooligosaccharides did not change after salt formation of chitooligosaccharides. Different salt chitooligosaccharides are completely different in promoting NO secretion by macrophages, and pure chitooligosaccharides are the best.

Soft Materials by Design: Unconventional Polymer Networks Give Extreme Properties

Hydrogels are polymer networks infiltrated with water. Many biological hydrogels in animal bodies such as muscles, heart valves, cartilages, and tendons possess extreme mechanical properties including being extremely tough, strong, resilient, adhesive, and fatigue-resistant. These mechanical properties are also critical for hydrogels' diverse applications ranging from drug delivery, tissue engineering, medical implants, wound dressings, and contact lenses to sensors, actuators, electronic devices, optical devices, batteries, water harvesters, and soft robots. Whereas numerous hydrogels have been developed over the last few decades, a set of general principles that can rationally guide the design of hydrogels using different materials and fabrication methods for various applications remain a central need in the field of soft materials. This review is aimed at synergistically reporting: (i) general design principles for hydrogels to achieve extreme mechanical and physical properties, (ii) implementation strategies for the design principles using unconventional polymer networks, and (iii) future directions for the orthogonal design of hydrogels to achieve multiple combined mechanical, physical, chemical, and biological properties. Because these design principles and implementation strategies are based on generic polymer networks, they are also applicable to other soft materials including elastomers and organogels. Overall, the review will not only provide comprehensive and systematic guidelines on the rational design of soft materials, but also provoke interdisciplinary discussions on a fundamental question: why does nature select soft materials with unconventional polymer networks to constitute the major parts of animal bodies?

Bionic chitosan-carbon imprinted aerogel for high selective recovery of Gd(Ⅲ) from end-of-life rare earth productions

High selective recovery of Gd(Ⅲ) from end-of-life rare earth productions is essential for cleaner production. Chitosan(CS), a biomaterial, has shown excellent results in water treatment. The amino and hydroxyl groups on the surface of CS play a vital role in adsorbing metal ions. Polydopamine has good stability, strong water dispersibility, and excellent biocompatibility. As a bio-crosslinking agent, the amino and phenolic hydroxyl groups on its surface can be combined with metal ions to help the material absorb metal ions. This paper combines the active groups of biomimetic materials and the mechanical properties of new nanomaterials multi-walled carbon nanotubes and graphene oxide, and prepared a high-performance chitosan-based aerogel MWCNT-PDA-CS-GO through heat and mass transfer at low temperature and low pressure. The adsorption mechanism of MWCNT-PDA-CS-GO for Gd(Ⅲ) was analyzed through a series of characterization and adsorption experiments. At pH 7.0, the maximum adsorption capacity of aerogel for Gd(Ⅲ) reached 150.86 mg g^-1. The relative selectivity of imprinted ions is 48.02 times higher than other ions. All the results indict MWCNT-PDA-CS-GO aerogel exhibits excellent selectivity and stability for effective recovery of Gd(Ⅲ).

Biosynthesis and characterization of deuterated chitosan in filamentous fungus and yeast

Deuterated chitosan was produced from the filamentous fungus Rhizopus oryzae, cultivated with deuterated glucose in H₂O medium, without the need for conventional chemical deacetylation. After extraction and purification, the chemical composition and structure were determined by Fourier-transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), and small-angle neutron scattering (SANS). ¹³C NMR experiments provided additional information about the position of the deuterons in the glucoseamine backbone. The NMR spectra indicated that the deuterium incorporation at the non-exchangeable hydrogen positions of the aminoglucopyranosyl ring in the C3 - C5 positions was at least 60-80 %. However, the C2 position was deuterated at a much lower level (6%). Also, SANS showed that the structure of deuterated chitosan was very similar compared to the non-deuterated counterpart. The most abundant radii of the protiated and deuterated chitosan fibers were 54 Å and 60 Å, respectively, but there is a broader distribution of fiber radii in the protiated chitosan sample. The highly deuterated, soluble fungal chitosan described here can be used as a model material for studying chitosan-enzyme complexes for future neutron scattering studies. Because the physical behavior of non-deuterated fungal chitosan mimicked that of shrimp shell chitosan, the methods presented here represent a new approach to producing a high quality deuterated non-animal-derived aminopolysaccharide for studying the structure-function association of biocomposite materials in drug delivery, tissue engineering and other bioactive chitosan-based composites.

Facile production of three-dimensional chitosan fiber embedded with zinc oxide as recoverable photocatalyst for organic dye degradation

Herein we report on a facile and green strategy for continuous production of chitosan-zinc oxide fibers and then compare their photodegradation performance against three organic dyes (i.e., methylene blue (MB), methyl orange (MO) and Rhodamine B, respectively) under different lights. Chitosan-zinc hydrogel fibers (CS/Zn) with different zinc loadings are obtained by direct mixing of chitosan and zinc acetate solutions using a double-syringe injection device. The as-prepared CS/Zn fibers are then immersed into glutaraldehyde (GA) and sodium hydroxide solutions, respectively, and dried at T = 50 °C. The resultant CS/ZnO/GA fibers of ca. 617 μm in diameter are characterized using X-ray diffraction (XRD), thermogravimetric analysis and field emission scanning electron microscope (FE-SEM). XRD and FE-SEM data confirm that the CS/ZnO/GA fibers consist of a large amount of hexagonal wurtzite ZnO nanorods up to 550 nm in length, and exhibit three-dimensional interconnected macroporous architecture. Photodegradation results clearly show that the CS/ZnO/GA fibers are effective for the removal of organic dyes upon UV irradiation and can be easily recovered and reused for at least 6 consecutive cycles. Unlike most reported CS/ZnO nanocomposites, the current CS/ZnO/GA fiber shows a higher adsorption of cationic MB rather than anionic MO, the mechanism of which is proposed.

Chitosan-based smart hybrid materials: a physico-chemical perspective

Chitosan is one of the most studied cationic polysaccharides. Due to its unique characteristics of being water soluble, biocompatible, biodegradable, and non-toxic, this macromolecule is highly attractive for a broad range of applications. In addition, its complex behavior and the number of ways it interacts with different components in a system result in an astonishing variety of chitosan-based materials. Herein, we present recent advances in the field of chitosan-based materials from a physico-chemical perspective, with focus on aqueous mixtures with oppositely charged colloids, chitosan-based thin films, and nanocomposite systems. In this review, we focus our attention on the physico-chemical properties of chitosan-based materials, including solubility, mechanical resistance, barrier properties, and thermal behaviour, and provide a link to the chemical peculiarities of chitosan, such as its intrinsic low solubility, high rigidity, large charge separation, and strong tendency to form intra- and inter-molecular hydrogen bonds.

Gold(III) recovery from aqueous solutions by raw and modified chitosan: A review

Chitosan, a prestigious versatile biopolymer, has recently received considerable attention as a promising biosorbent for recovering gold ions, mainly Au(III), from aqueous solutions, particularly in modified forms. Confirming the assertion, this paper provides an up-to-date overview of Au(III) recovery from aqueous solutions by raw (unmodified) and modified chitosan. A particular emphasis is placed on the raw chitosan and its synthesis from chitin, characteristics of raw chitosan and their effects on metal sorption, modifications of raw chitosan for Au(III) sorption, and characterization of raw chitosan before and after modifications for Au(III) sorption. Comparisons of the sorption (conditions, percentage, capacity, selectivity, isotherms, thermodynamics, kinetics, and mechanisms), desorption (agents and percentage), and reusable properties between raw and modified chitosan in Au(III) recovery from aqueous solutions are also outlined and discussed. The major challenges and future prospects towards the large-scale applications of modified chitosan in Au(III) recovery from aqueous solutions are also addressed.

Injectable and Degradable PEG Hydrogel with Antibacterial Performance for Promoting Wound Healing

Injectable and degradable PEG hydrogel was prepared via Michael-type addition between cross-linking monomer 4-arm-PEG-MAL and two cross-linkers of hydrolysis degradable PEG-diester-dithiol and non-degradable PEG-dithiol, and it had a porous structure with the uniform pore size. The biocompatibility assays in vitro indicated that PEG hydrogel had excellent biocompatibility and can be degraded naturally without leading to any negative impact on cells. The results of antibacterial experiments showed that PEG hydrogel can inhibit the growth of bacteria. Furthermore, the Cell Counting Kit-8 (CCK-8) assay, LIVE/DEAD cell staining, and scratch healing experiments proved that PEG hydrogel can promote cell proliferation and migration, which had been further confirmed in in vivo experiments on the rat wound models. All experimental results demonstrated that PEG hydrogel is an injectable antibacterial dressing, which can promote the process of wound healing and has great potential in the field of wound healing.

Effect of Crosslinking Type on the Physical-Chemical Properties and Biocompatibility of Chitosan-Based Electrospun Membranes

Chitosan nanofibrous membranes are prepared via an electrospinning technique and explored as potential wound healing patches. In particular, the effect of a physical or chemical crosslinking treatment on the mat morphological, mechanical, water-related, and biological properties is deeply evaluated. The use of phosphate ions (i.e., physical crosslinking) allows us to obtain smooth and highly homogenous nanofibers with an average size of 190 nm, whereas the use of ethylene glycol diglycidyl ether (i.e., chemical crosslinking) leads to rougher, partially coalesced, and bigger nanofibers with an average dimension of 270 nm. Additionally, the physically crosslinked mats show enhanced mechanical performances, as well as greater water vapour permeability and hydrophilicity, with respect to the chemically crosslinked ones. Above all, cell adhesion and cytotoxicity experiments demonstrate that the use of phosphate ions as crosslinkers significantly improves the capability of chitosan mats to promote cell viability owing to their higher biocompatibility. Moreover, tuneable drug delivery properties are achieved for the physically crosslinked mats by a simple post-processing impregnation methodology, thereby indicating the possibility to enrich the prepared membranes with unique features. The results prove that the proposed approach may lead to the preparation of cheap, biocompatible, and efficient chitosan-based nanofibers for biomedical and pharmaceutical applications.

A Review on the Design and Hydration Properties of Natural Polymer-Based Hydrogels

Hydrogels are hydrophilic 3D networks that are able to ingest large amounts of water or biological fluids, and are potential candidates for biosensors, drug delivery vectors, energy harvester devices, and carriers or matrices for cells in tissue engineering. Natural polymers, e.g., cellulose, chitosan and starch, have excellent properties that afford fabrication of advanced hydrogel materials for biomedical applications: biodegradability, biocompatibility, non-toxicity, hydrophilicity, thermal and chemical stability, and the high capacity for swelling induced by facile synthetic modification, among other physicochemical properties. Hydrogels require variable time to reach an equilibrium swelling due to the variable diffusion rates of water sorption, capillary action, and other modalities. In this study, the nature, transport kinetics, and the role of water in the formation and structural stability of various types of hydrogels comprised of natural polymers are reviewed. Since water is an integral part of hydrogels that constitute a substantive portion of its composition, there is a need to obtain an improved understanding of the role of hydration in the structure, degree of swelling and the mechanical stability of such biomaterial hydrogels. The capacity of the polymer chains to swell in an aqueous solvent can be expressed by the rubber elasticity theory and other thermodynamic contributions; whereas the rate of water diffusion can be driven either by concentration gradient or chemical potential. An overview of fabrication strategies for various types of hydrogels is presented as well as their responsiveness to external stimuli, along with their potential utility in diverse and novel applications. This review aims to shed light on the role of hydration to the structure and function of hydrogels. In turn, this review will further contribute to the development of advanced materials, such as "injectable hydrogels" and super-adsorbents for applications in the field of environmental science and biomedicine.

Effect of Varying Amount of Polyethylene Glycol (PEG-600) and 3-Aminopropyltriethoxysilane on the Properties of Chitosan based Reverse Osmosis Membranes

Chitosan and polyethylene glycol (PEG-600) membranes were synthesized and crosslinked with 3-aminopropyltriethoxysilane (APTES). The main purpose of this research work is to synthesize RO membranes which can be used to provide desalinated water for drinking, industrial and agricultural purposes. Hydrogen bonding between chitosan and PEG was confirmed by displacement of the hydroxyl absorption peak at 3237 cm-1 in pure chitosan to lower values in crosslinked membranes by using FTIR. Dynamic mechanical analysis revealed that PEG lowers Tg of the modified membranes vs. pure chitosan from 128.5 °C in control to 120 °C in CS-PEG5. SEM results highlighted porous and anisotropic structure of crosslinked membranes. As the amount of PEG was increased, hydrophilicity of membranes was increased and water absorption increased up to a maximum of 67.34%. Permeation data showed that flux and salt rejection value of the modified membranes was increased up to a maximum of 80% and 40.4%, respectively. Modified films have antibacterial properties against Escherichia coli as compared to control membranes.

Construction of natural polymeric imprinted materials and their applications in water treatment: A review

Molecularly imprinted materials (MIMs) have been widely used in various fields, including water treatment, chemical sensing, and biotechnology, because of their specific recognition and high selectivity. MIMs are usually obtained via two successive steps, namely, (1) copolymerization and crosslinking reactions of the preassembled complex of comonomers and a specific target compound (2) and thorough removal of template molecules. Some functional polymers are directly used as supporting materials and functional groups assembled with target compound are provided to simplify the preparation of MIMs. Natural polymers, such as chitosan, cyclodextrin, sodium alginate, starch, cellulose, lignin and their derivatives, are good candidates because of their environmentally friendly properties, low costs, and abundant active functional groups. In this study, different methods for the preparation of natural polymeric MIMs were reviewed in terms of the construction of microscopic binding cavities and macroscopic visible condensed structures with different shapes. Natural polymeric MIMs in water treatment applications, such as adsorption and detection of various pollutants from aqueous solutions, were summarized. Prospects on the development of novel and high-performance natural polymeric MIMs were discussed to overcome the difficulties in their preparation and applications.

Optimization of Chitosan Glutaraldehyde-Crosslinked Beads for Reactive Blue 4 Anionic Dye Removal Using a Surface Response Methodology

The use of dyes at an industrial level has become problematic, since the discharge of dye effluents into water disturbs the photosynthetic activity of numerous aquatic organisms by reducing the penetration of light and oxygen, in addition to causing carcinogenic diseases and mutagenic effects in humans, as well as alterations in different ecosystems. Chitosan (CS) is suitable for removing anionic dyes since it has favorable properties, such as acquiring a positive charge and a typical macromolecular structure of polysaccharides. In this study, the optimization of CS beads crosslinked with glutaraldehyde (GA) for the adsorption of reactive blue dye 4 (RB4) in an aqueous solution was carried out. In this sense, the response surface methodology (RSM) was applied to evaluate the concentration of CS, GA, and sodium hydroxide on the swelling degree in the GA-crosslinked CS beads. In the same way, RSM was applied to optimize the adsorption process of the RB4 dye as a function of the initial pH of the solution, initial concentration of the dye, and adsorbent dose. The crosslinking reaction was investigated by scanning electron microscopy (SEM), Fourier transformed infrared spectroscopy (FTIR), and X-ray diffractometry (XRD). The design described for the swelling degree showed an R2 (coefficient of determination) adjusted of 0.8634 and optimized concentrations (CS 3.3% w/v, GA 1.7% v/v, and NaOH 1.3 M) that were conveniently applied with a concentration of CS at 3.0% w/v to decrease the viscosity and facilitate the formation of the beads. In the RB4 dye adsorption design, an adjusted R2 (0.8280) with good correlation was observed, where the optimized conditions were: pH = 2, adsorbent dose 0.6 g, and initial concentration of RB4 dye 5 mg/L. The kinetic behavior and the adsorption isotherm allowed us to conclude that the GA-crosslinked CS beads' adsorption mechanism was controlled mainly by chemisorption interactions, demonstrating its applicability in systems that require the removal of contaminants with similar structures to the model presented.

Recent Developments in Chitosan-Based Adsorbents for the Removal of Pollutants from Aqueous Environments

The quality of water is continuously under threat as increasing concentrations of pollutants escape into the aquatic environment. However, these issues can be alleviated by adsorbing pollutants onto adsorbents. Chitosan and its composites are attracting considerable interest as environmentally acceptable adsorbents and have the potential to remove many of these contaminants. In this review the development of chitosan-based adsorbents is described and discussed. Following a short introduction to the extraction of chitin from seafood wastes, followed by its conversion to chitosan, the properties of chitosan are described. Then, the emerging chitosan/carbon-based materials, including magnetic chitosan and chitosan combined with graphene oxide, carbon nanotubes, biochar, and activated carbon and also chitosan-silica composites are introduced. The applications of these materials in the removal of various heavy metal ions, including Cr(VI), Pb(II), Cd(II), Cu(II), and different cationic and anionic dyes, phenol and other organic molecules, such as antibiotics, are reviewed, compared and discussed. Adsorption isotherms and adsorption kinetics are then highlighted and followed by details on the mechanisms of adsorption and the role of the chitosan and the carbon or silica supports. Based on the reviewed papers, it is clear, that while some challenges remain, chitosan-based materials are emerging as promising adsorbents.

Green synthesis of nano-Al2O3, recent functionalization, and fabrication of synthetic or natural polymer nanocomposites: various technological applications

Environmentally friendly fabrication of nano-Al₂O₃, recent functionalization, and preparation of polymer nanocomposites including natural and man-made polymers with various industrial applications are reviewed.

Terephthalaldehyde as a good crosslinking agent in crosslinked chitosan hydrogel for the selective removal of anionic dyes

This work reports the selectivity and good adsorption performance for the removal of anionic dyes using an ultrasonic-synthesized terephthalaldehyde crosslinked chitosan hydrogel.

Arsenate Adsorption on Fly Ash, Chitosan and Their Composites and Its Relations with Surface, Charge and Pore Properties of the Sorbents

One of the ways to recycle millions of tons of fly ash and chitin wastes produced yearly is their utilization as low-cost sorbents, mainly for heavy metal cations and organic substances. To improve their sorption efficiency, fly ashes have been thermally activated or modified by chitosan. We aimed to deeply characterize the physicochemical properties of such sorbents to reveal the usefulness of modification procedures and their effect on As(V) adsorption. Using low temperature nitrogen adsorption, scanning electron microscopy, mercury intrusion porosimetry, potentiometric titration and adsorption isotherms of As(V) anions, surface, pore, charge and anion adsorption parameters of fly ash activated at various temperatures, chitosan, and fly ash modified by chitosan were determined. Arsenate adsorption equilibrium (Langmuir model), kinetics (pseudo-second order model) and thermodynamics on the obtained materials were studied. Neither temperature activation nor chitosan modifications of fly ash were necessary and profitable for improving physicochemical properties and As(V) adsorption efficiency of fly ash. Practically, the physicochemical parameters of the sorbents were not related to their anion adsorption parameters.

Nanoscale mechanical properties of chitosan hydrogels as revealed by AFM

In the context of tissue engineering, chitosan hydrogels are attractive biomaterials because they represent a family of natural polymers exhibiting several suitable features (cytocompatibility, bioresorbability, wound healing, bacteriostatic and fungistatic properties, structural similarity with glycosaminoglycans), and tunable mechanical properties. Optimizing the design of these biomaterials requires fine knowledge of its physical characteristics prior to assessment of the cell-biomaterial interactions. In this work, using atomic force microscopy (AFM), we report a characterization of mechanical and topographical properties at the submicron range of chitosan hydrogels, depending on physico-chemical parameters such as their polymer concentration (1.5%, 2.5% and 3.5%), their degree of acetylation (4% and 38.5%), and the conditions of the gelation process. Well-known polyacrylamide gels were used to validate the methodology approach for the determination and analysis of elastic modulus (i.e., Young's modulus) distribution at the gel surface. We present elastic modulus distribution and topographical and stiffness maps for different chitosan hydrogels. For each chitosan hydrogel formulation, AFM analyses reveal a specific asymmetric elastic modulus distribution that constitutes a useful hallmark for chitosan hydrogel characterization. Our results regarding the local mechanical properties and the topography of chitosan hydrogels initiate new possibilities for an interpretation of the behavior of cells in contact with such soft materials.