An environmentally-friendly chitosan-lysozyme biocomposite for the effective removal of dyes and heavy metals from aqueous solutions

Adsorption of contaminants by nanomaterials synthesized by green and conventional routes: a critical review

Nanomaterials, due to their large surface area and selectivity, have stood out as an alternative for the adsorption of contaminants from water and effluents. Synthesized from green or traditional protocols, the main advantages and disadvantages of green nanomaterials are the elimination of the use of toxic chemicals and difficulty of reproducing the preparation of nanomaterials, respectively, while traditional nanomaterials have the main advantage of being able to prepare nanomaterials with well-defined morphological properties and the disadvantage of using potentially toxic chemicals. Thus, based on the particularities of green and conventional nanomaterials, this review aims to fill a gap in the literature on the comparison of the synthesis, morphology, and application of these nanomaterials in the adsorption of contaminants in water. Focusing on the adsorption of heavy metals, pesticides, pharmaceuticals, dyes, polyaromatic hydrocarbons, and phenol derivatives in water, for the first time, a review article explored and compared how chemical and morphological changes in nanoadsorbents synthesized by green and conventional protocols affect performance in the adsorption of contaminants in water. Despite advances in the area, there is still a lack of review articles on the topic.

Preparation and characterization of lysozyme loaded cryogel for heavy metal removal

In the present study, monolithic poly(N-isopropylacrylamide-acrylamide)-acrilic acid (poly(npam-aam)-aac) cryogels were made. Swelling tests, SEM, XRD, and ATR-FTIR analyses revealed distinct cryogel and lysozyme-loaded cryogel properties. The equilibrium swelling degree was 6.2 g H2O/g cryogel. The created poly(npam-aam)-aac with pores of 10-100 μm was obviously seen in SEM images. Lysozyme adsorption capacity on poly(npam-aam)-aac was found to be 260 mg/g at pH 7.4 and 40 °C. After that, we used lysozyme adsorbed cryogel for the removal of the model heavy metal ion (cadmium). A series of pH, duration, and ionic strengths were used to conduct Cd2+ adsorption experiments. The results showed that the new adsorbent had a considerable chemical affinity for Cd2+ ions in its ability to bind them under eye ocular conditions (pH 7.4, 32-36 °C, 0,15 M NaCl). The traditional Langmuir adsorption model was the most suitable, achieving maximum uptake of ∼185 mg/g. Chemical adsorption was found to be the rate-controlling step, and the process was also compatible with the pseudo-second-order model. For the treatment of ocular pathologies, the most effective enzyme, lysozyme, must show its function. That is why there is a need for using lysozyme, and lysozyme is selected as a lignad to adsorb heavy metal ions because of its high heavy metal binding affinity. This material could be used for the treatment of ocular pathologies in the future.

A review on heavy metal biosorption utilizing modified chitosan

Heavy metal pollution in water bodies is a global concern. The prominent source of metal contamination in aqueous streams and groundwater is wastewater containing heavy metal ions. Elevated concentrations of heavy metals in water bodies can have a negative impact on water quality and public health. The most effective way to remove metal contaminants from drinking water is thought to be adsorption. A deacetylated derivative of chitin, chitosan, has a wide range of commercial uses since it is biocompatible, nontoxic, and biodegradable. Due to its exceptional adsorption behavior toward numerous hazardous heavy metals from aqueous solutions, chitosan and its modifications have drawn a lot of interest in recent years. Due to its remarkable adsorption behavior toward a range of dangerous heavy metals, chitosan is a possible agent for eliminating metals from aqueous solutions. The review has focused on the ideas of biosorption, its kinds, architectures, and characteristics, as well as using modified (physically and chemically modified) chitosan, blends, and composites to remove heavy metals from water. The main objective of the review is to describe the most important aspects of chitosan-based adsorbents that might be beneficial for enhancing the adsorption capabilities of modified chitosan and promoting the usage of this material in the removal of heavy metal pollutants.

Regenerated cellulose/chitosan composite aerogel with highly efficient adsorption for anionic dyes

A novel reusable, high-compressible cotton regenerated cellulose/chitosan composite aerogel (RC/CSCA) was prepared using N-methylmorpholine-N-oxide (NMMO) as the green cellulose solvent, and glutaraldehyde (GA) as the crosslinking agent. The regenerated cellulose obtained from cotton pulp could chemically crosslink with chitosan and GA, to form a stable 3D porous structure. The GA played an essential role in preventing shrinkage and preserving the deformation recovery ability of RC/CSCA. Due to the ultralow density (13.92 mg/cm³), thermal stability (above 300 °C), and high porosity (97.36 %), the positively charged RC/CSCA can be used as a novel biocomposite adsorbent for effective and selective removal of toxic anionic dyes from wastewater, showing an excellent adsorption capacity, environmental adaptability, and recyclability. The maximal adsorption capacity and removal efficiency of RC/CSCA for methyl orange (MO) was 742.68 mg/g and 95.83 %.

The use of chitosan-based composites for environmental remediation: A review

The presence of hazardous pollutants in water sources as a result of industrial activities is a major environmental challenge that impedes the availability of safe drinking water. Adsorptive and photocatalytic degradative removal of various pollutants in wastewater have been recognized as cost-effective and energy-efficient strategies. In addition to its biological activity, chitosan and its derivatives are considered as promising materials for the removal of various pollutants. The abundance of hydroxyl and amino groups in the chitosan macromolecular structure results in a variety of concurrent pollutant's adsorption mechanisms. Furthermore, adding chitosan to photocatalysts increases the mass transfer while decreasing both the band gap energy and the amount of intermediates produced during photocatalytic processes, improving the overall photocatalytic efficiency. Herein, we have reviewed the current design and preparation of chitosan and its composites, as well as their applications for the removal of various pollutants by adsorption and photocatalysis processes. Effects of operating variables such as the pH, catalyst mass, contact time, light wavelength, initial pollutant's concentration, and catalyst recyclability, are discussed. Various kinetic and isotherm models are presented to elucidate the rates, and mechanisms of pollutant's removal, onto chitosan-based composites, and several case studies are presented. Additionally, the antibacterial activity of chitosan-based composites has been discussed. This review aims to provide a comprehensive and up-to-date overview of the applications of chitosan-based composites in wastewater treatment and put forward new insights for the development of highly effective chitosan-based adsorbents and photocatalysts. Finally, the main challenges and future directions in the field are discussed.

Preparation of metal organic frameworks modified chitosan composite with high capacity for Hg(II) adsorption

In this study, a novel modified chitosan composite adsorbent (UNCS) was prepared by crosslinking between chitosan and metal organic frameworks (MOFs) material UiO-66-NH₂ using epichlorohydrin as crosslinker. The influence of the prepared conditions was investigated. The structure and morphology of the composite were characterized by FT-IR, XRD, SEM, TGA, BET and zeta potential analysis. Effects of different variables for adsorption of Hg(II) on this adsorbent were explored. The kinetic studies indicated that the adsorption process followed the pseudo-second-order kinetic model and the adsorption equilibrium could be reached within 2 h. The adsorption was mainly controlled by chemical process. Adsorption isothermal studies illustrated that the adsorption fitted Langmuir isotherm model, implying the homogeneous adsorption on the surface of the adsorbent. The adsorbent exhibited high uptake and the maximum capacity from Langmuir model could reach 896.8 mg g^-1 at pH 6. Thermodynamic studies showed the spontaneous nature and exothermic nature of the adsorption process. Additionally, the removal of Hg(II) on UNCS could achieve over 90 %. The adsorption-desorption cycled experiments indicated the appropriate reusability of the adsorbent. Hence, this adsorbent would be promising for the removal of Hg(II) from wastewater.

A Novel Chitosan/Nano-Hydroxyapatite Composite for the Adsorptive Removal of Cd(II) from Aqueous Solution

A novel polymer bio-composite based on nano-hydroxyapatite (n-Hap) and chitosan (CS) (CS/n-Hap) was synthesized to effectively address toxic cadmium ions removal from water. The composition and structure of CS/n-Hap bio-composite were analyzed through different characterization techniques. XRD patterns affirmed that the crystalline structure of n-Hap remained unaltered during CS/n-Hap synthesis, while FT-IR spectrum sustained all the characteristic peaks of both CS and n-Hap, affirming the successful synthesis of CS/n-Hap. Adsorption studies, including pH, adsorbent dosage, contact time, initial Cd(II) concentration, and temperature, were carried out to explain and understand the adsorption mechanism. Comparatively, CS/n-Hap bio-composite exhibited better Cd(II) adsorption capacity than pristine CS, with an experimental maximum uptake of 126.65 mg/g under optimized conditions. In addition, the kinetic data were well fitted to the pseudo-second-order model, indicating the formation of chemical bonds between Cd(II) and CS/n-Hap during adsorption. Furthermore, the thermodynamic study suggested that Cd(II) adsorption onto CS/n-Hap was endothermic and spontaneous. The regeneration study showed only about a 3% loss in Cd(II) uptake by CS/n-Hap after five consecutive cycles. Thus, a simple and facile approach was here developed to synthesize an eco-friendly and cost-effective material that can be successfully employed for the removal of toxic heavy metal ions from water.

A pyridinium functionalization chitosan for efficient elimination of methyl orange and Cr(VI)

A pyridinium functionalization chitosan (PCS) at high yield was facilely and solvothermally obtained from reactions of chitosan with N-2,4-dinitrophenyl-pyridinium chloride. The morphology and physical-chemical properties of PCS were tested with various techniques. Its sorption behaviors towards methyl orange (MO) and Cr(VI) were systematically investigated. Pseudo-second-order kinetic and Langmuir equations well fitted the sorption kinetics and isotherms, respectively. Thermodynamics analyses revealed the spontaneous and endothermic sorption of these two contaminants. PCS exhibited high sorption capacity of 1649.30 mg·g^-1 MO and 200.46 mg·g^-1 Cr(VI) at 308 K. The superior sorption performance of PCS over MO is ascribed to ion exchange, intermolecular hydrogen bond, and electrostatic and π-π interactions, while sorption of PCS over Cr(VI) is mainly driven by electrostatic forces, reduction and ion exchange. Moreover, the PCSexceeded 95 % of its original capacities during five cycles. This high sorption capacities and high reusability make PCS an excellent sorbent candidate towards anionic contaminants.

Adsorption and Removal of Mercury(II) by a Crosslinked Hyperbranched Polymer Modified via Sulfhydryl

In this study, the highly crosslinked hyperbranched polyamide-amines (H-PAMAMs) were first prepared via one-pot methods and then modified with thiourea to synthesize a novel adsorbent containing sulfhydryl groups (CHAP-SH), which was used to adsorb Hg(II) ions from aqueous solutions. The adsorption characteristics and mechanism of CHAP-SH for Hg(II) ions were systematically studied. As expected, CHAP-SH exhibited a rapid removal performance toward Hg(II), and the maximum adsorption capacity was 282.74 mg/g at 318 K and pH = 4.5. The whole adsorption behavior could be well described by the pseudo-second-order kinetic model and Langmuir and Redlich-Peterson adsorption isotherm models, which reflected that the adsorption process was mainly monolayer chemisorption. Meanwhile, CHAP-SH had strong selectivity for Hg(II) in the presence of multimetal ions, and it had excellent recoverability after five cycles. In order to further elucidate the adsorption mechanism, the adsorbents before and after adsorption were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis, and energy-dispersive X-ray spectroscopy, and the results showed that the nitrogen-containing, oxygen-containing, and sulfur-containing groups in the adsorbent molecule had synergistic complexation with Hg(II). These results indicated that the adsorbents had great potential in the future treatment of aqueous solutions containing Hg(II).

Chitosan as a Tool for Sustainable Development: A Mini Review

New developments require innovative ecofriendly materials defined by their biocompatibility, biodegradability, and versatility. For that reason, the scientific society is focused on biopolymers such as chitosan, which is the second most abundant in the world after cellulose. These new materials should show good properties in terms of sustainability, circularity, and energy consumption during industrial applications. The idea is to replace traditional raw materials with new ecofriendly materials which contribute to keeping a high production rate but also reducing its environmental impact and the costs. The chitosan shows interesting and unique properties, thus it can be used for different purposes which contributes to the design and development of sustainable novel materials. This helps in promoting sustainability through the use of chitosan and diverse materials based on it. For example, it is a good sustainable alternative for food packaging or it can be used for sustainable agriculture. The chitosan can also reduce the pollution of other industrial processes such as paper production. This mini review collects some of the most important advances for the sustainable use of chitosan for promoting circular economy. Hence, the present review focuses on different aspects of chitosan from its synthesis to multiple applications.

Proteinaceous porous nanofiber membrane-type adsorbent derived from amyloid lysozyme protofilaments for highly efficient lead(II) biologic scavenging

To overcome the technical bottleneck of fine amyloid lysozyme fibrils in environmental engineering, a novel co-operative strategy was identified to fabricate free-standing lysozyme complex nanofibers based membrane-type adsorbent (Lys-CNFs membrane) through a combination of vacuum filtration for lead remediation. The composition of the membrane integrated the linear amyloid protofilaments that were obtained by acid-heating fibrillation and polydopamine that adjusted the fibers' diameters and surface chemistry. As expected, the Lys-CNFs membrane not only showed nanofibrous morphology and layer stacking architecture but presented a hierarchical macro-mesoporous structure along with a high surface area of 220.4 m²/g. Besides, the thermal stability up to 200 ℃ and wetting nature of below 2 s endowed its further applicability. Adsorption experiments showed that Lys-CNFs membrane can effectively uptake Pb(II) ions with acceptable selectivity, high adsorption capacity of 270.3 mg/g, rapid equilibrium kinetic within only 10 mins, and good reusability that dropped by 14.9% efficiency even after five cycles, indicating that Lys-CNFs membrane can be as an affordable technology for alleviating the lead pollution issues.

A versatile EDTA and chitosan bi-functionalized magnetic bamboo biochar for simultaneous removal of methyl orange and heavy metals from complex wastewater

At present, the simultaneous removal of organic dyes and heavy metals in complex wastewater has raised considerable concern, owing to their striking differences in physicochemical properties. Adsorption, as one of the few removal methods, has attracted extensive attention and gained popularity. Herein, a versatile EDTA and chitosan bi-functionalized magnetic bamboo biochar adsorbent (ECMBB) was synthesized for coinstantaneous adsorption of methyl orange (MO) and heavy metals (Cd(II) and Zn(II)). In this case, the as-synthesized ECMBB composites inherited favorable anionic MO removal performance from bamboo biochar (BB) obtained at 700 °C through electrostatic attraction, hydrogen bonding and π-π interaction, also enhanced the binding of cationic metals by introducing amino groups of chitosan and carboxyl groups of EDTA. In the unitary system, the removal of MO, Cd(II) and Zn(II) by three as-prepared adsorbents can be well illuminated by pseudo-second-order kinetic model and Langmuir isotherm theory. The saturated capture amounts of ECMBB at 25 °C are 305.4 mg g^-1 for MO, 63.2 mg g^-1 for Cd(II) and 50.8 mg g^-1 for Zn(II), which, under the same conditions, are 1.3, 2.6 and 2.5 times those of chitosan-modified magnetic bamboo biochar (CMBB) and 1.9, 6.1 and 5.4 times those of magnetic bamboo biochar (MBB), respectively. Remarkably, in MO-metal binary system, coexisting MO visibly enhanced the adsorption of Cd(II) and Zn(II), while coexisting heavy metals had no significant impact on MO adsorption. Furthermore, ECMBB exhibited no significant loss in adsorption efficiency even after eight adsorption-desorption experiments. This study lays the foundation for fabricating desired integrative biochar adsorbents in the simultaneous purification of organic and metallic pollutants from complex wastewater.

Sustainable Development of Magnetic Chitosan Core-Shell Network for the Removal of Organic Dyes from Aqueous Solutions

The wide use of alizarin red S (ARS), a typical anthraquinone dye, has led to its continued accumulation in the aquatic environment, which causes mutagenic and carcinogenic effects on organisms. Therefore, this study focused on the removal of ARS dye by adsorption onto a magnetic chitosan core-shell network (MCN). The successful synthesis of the MCN was confirmed by ATR-FTIR, SEM, and EDX analysis. The influence of several parameters on the removal of ARS dye by the MCN revealed that the adsorption process reached equilibrium after 60 min, pH played a major role, and electrostatic interactions dominated for the ARS dye removal under acidic conditions. The adsorption data were described well by the Langmuir isotherm and a pseudo-second order kinetic model. In addition to the preferable adsorption of hydrophobic dissolved organic matter (DOM) fractions onto the MCN, the electrostatic repulsive forces between the previously adsorbed DOM onto MCN and ARS dye resulted in lower ARS dye removal. Furthermore, the MCN could easily be regenerated and reused for up to at least five cycles with more than 70% of its original efficiency. Most importantly, the spent MCN was pyrolytically converted into N-doped magnetic carbon and used as an adsorbent for various dyes, thus establishing a waste-free adsorption process.

Applications of chitin and chitosan based biomaterials for the adsorptive removal of textile dyes from water - A comprehensive review

The presence of pollutants in the water bodies deteriorate the water quality and make it unfit for use. From an environmental perspective, it is essential to develop new technologies for the wastewater treatment and recycling of dye contaminated water. The surface modified chitin and chitosan biopolymeric composites based adsorbents, have an important role in the toxic organic dyes from removal wastewater. The surface modification of biopolymers with various organics and inorganics produces more active sites at the surface of the adsorbent, which enhances dye and adsorbent interaction more reliable. Herein, the work brought in the thought of the application of various chitin and chitosan composites in wastewater remediation and suggested the versatility in composites for the development of rapid, selective and effective removal processes for the detoxification of a variety of organic dyes. It further emphasizes the existing obstruction and impending prediction for the deprivation of dyes via adsorption techniques.

Critical review on hazardous pollutants in water environment: Occurrence, monitoring, fate, removal technologies and risk assessment

Water is required for the existence of all living things. Water pollution has grown significantly, over the decades and now it has developed as a serious worldwide problem. The presence and persistence of Hazardous pollutants such as dyes, pharmaceuticals and personal care products, heavy metals, fertilizer and pesticides and their transformed products are the matter of serious environmental and health concerns. A variety of approaches have been tried to clean up water and maintain water quality. The type of pollutants present in the water determines the bulk of technological solutions. The main objective of this article was to review the occurrences and fate of hazardous contaminants (dyes, pharmaceuticals and personal care products, heavy metals, and pesticides) found in wastewater effluents. These effluents mingle with other streams of water and that are utilized for a variety of reasons such as irrigation and other domestic activities that is further complicating the issue. It also discussed traditional treatment approaches as well as current advances in hazardous pollutants removal employing graphite oxides, carbon nanotubes, metal organic structures, magnetic nano composites, and other innovative forms of useable materials. It also discussed the identification and quantification of harmful pollutants using various approaches, as well as current advancements. Finally, a risk assessment of hazardous pollutants in water is provided in terms of the human health and the environment. This data is anticipated to serve as a foundation for future improvements in hazardous pollutant risk assessment. Furthermore, future studies on hazardous pollutants must not only emphasize on the parent chemicals, as well as on their possible breakdown products in various media.

Enhanced sequestration of Cr(VI) onto plant extract anchored on carbon-coated aluminium oxide composite

Aluminium oxide (ALU) and carbon-coated aluminium oxide modified with Kigelia africana leaf extract (KECA) were employed for the removal of toxic hexavalent chromium (Cr(VI)) from the aqueous phase. The adsorbents (ALU and KECA) were characterized by TGA, BET, FESEM, FTIR, Raman and XRD spectroscopic techniques. The potential of KECA and ALU to remove Cr(VI) from simulated wastewater was optimum at pH 2, sorbent dose of 0.025 g and a contact time of 200 min. Meanwhile, the uptake capacity of KECA and ALU was enhanced with an increase in sorbent dose, contact time and initial Cr(VI) concentration. The uptake of Cr(VI) onto the adsorbents ALU and KECA was kinetically best described by the pseudo-second-order and Elovich models, respectively. Besides, the equilibrium data acquired for ALU and KECA obeyed Freundlich and Langmuir isotherm models, respectively. ALU and KECA were observed to have optimum adsorption capacity of 56.45 mg g^-1 and 258.2 mg g^-1, respectively. The adsorption of Cr(VI) onto the adsorbents was thermodynamically feasible, endothermic in nature and entropy-driven. A decrease in efficiency was observed on regeneration of the absorbents, thus limiting their reusability. However, the presence of functional groups with reducing property in the extract of Kigelia africana leaves was noticed to enhance the capacity of the adsorbent to abstract Cr(VI) from the solution. Hence, this study demonstrates the potential of KECA to sequestrate Cr(VI) from an aqueous solution and provides a reference for its application to the treatment of Cr(VI)-laden industrial wastewater.

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.

Research progress of adsorption and removal of heavy metals by chitosan and its derivatives: A review

Chitosan has received widespread attention as an adsorbent for pollutants because of its low cost and great adsorption potentials. Chitosan has abundant hydroxyl and amino groups that can bind heavy metal ions. However, it has defects such as sensitivity to pH, low thermal stability, and low mechanical strength, which limit the application of chitosan in wastewater treatment. The functional groups of chitosan can be modified to improve its performance via crosslinking and graft modification. The porosity and specific surface area of chitosan in powder form are not ideal, therefore, physical modification has been attempted to generate chitosan nanoparticles and hydrogel. Chitosan has also been integrated with other materials (e.g. graphene, zeolite) resulting in composite materials with improved adsorption performance. This review mainly focuses on reports about the application of chitosan and its derivatives to remove different heavy metals. The preparation strategy, adsorption mechanism, and factors affecting the adsorption performance of adsorbents for each type of heavy metal are discussed in detail. Recent reports on important organic pollutants (dyes and phenol) removal by chitosan and its derivatives are also briefly discussed.

Hydrothermal synthesis of hydroxyapatite-reduced graphene oxide (1D-2D) hybrids with enhanced selective adsorption properties for methyl orange and hexavalent chromium from aqueous solutions

The presence of organic dye molecules and heavy metal ions in water causes ecological and public health problems. Therefore, remediation of water/wastewater contaminated with organic dye molecules and toxic metal ions is of importance. Herein, a reduced graphene oxide (RGO)-hydroxyapatite (Hat) (1D-2D) hybrid composite was fabricated through a hydrothermal process and applied for the adsorption of methyl orange (MO) and hexavalent chromium (Cr(VI)) from water. The as-fabricated RGO-Hat hybrids were characterized using FTIR, XRD, HR-TEM, SEM, XPS, EDAX, and TGA-DSC analytical techniques. The influencing parameters of adsorption performance, namely solution pH, contact time, and co-interfering ions, were explored to obtain the maximum adsorption capacity of contaminants from the solid-liquid interface. Batch studies revealed that MO and Cr(VI) adsorption followed the pseudo-second-order kinetic and the Langmuir isotherm models. The adsorption capacity was 49.14 and 45.24 mg g^-1 for MO and Cr(VI), respectively. The adsorption of such ions over RGO-Hat hybrids was mainly driven by several uptake mechanisms viz, electrostatic force of attraction, π-π interactions, and hydrogen bonding. Thus, this study demonstrated that the RGO-Hat hybrid is a potential candidate for the treatment of MO and Cr(VI) from water.

A review on cleaner strategies for extraction of chitosan and its application in toxic pollutant removal

Existence of human beings in this world require a cleaner environment, in which, water is the main requirement for living. Owing to the considerable development in civilisation and considerable population explosion, an increase in the contamination of natural water resources by means of non-biodegradable contaminants like heavy metals is observed thereby increasing the need for treatment of water before usage. Despite the existence of specific limits for disposal of heavy metals in water resources, studies still show high contamination of heavy metals in all these water resources. This review provides a brief note on sources and toxicity of different heavy metals in various oxidation states, their effects as well as highlights the numerous available and advanced techniques for heavy metals removal. Of all techniques adsorption is found to be beneficial as it doesn't inculcate any secondary pollutants to the environment. Additionally, this article has investigated the advantages of polymer nanocomposites in adsorption and mainly focused on biopolymer chitosan owing to its abundance in natural environment. The cleaner techniques for the extraction of chitosan and its functionalisation using different types of nanofillers are comprehensively discussed in this review. This article suggests a better alternative for conventional adsorbents as well as aids in remediation of wastes.

Chitosan-based hybrid materials for adsorptive removal of dyes and underlying interaction mechanisms

Environmental pollution by dyes molecules has become a subject of intensive research in recent years due to their hazardous effects on human health, organisms, and animals. Effective treatment and removal of dye molecules from the environmental matrices and water sources are of supreme concern. The deployment of cheap, safe, green, sustainable, and eco-friendly materials to remove these pollutants from water is the main challenge during the last decades. Chitosan and its derivatives/composites, as a cheap, easily available, and environmentally friendly sorbent, have attracted increasing attention for the removal of dye molecules. This review article focuses on the application of chitosan and chitosan-based smart adsorbents for the removal of dyes. Recent methods for the preparation of chitosan-based composites and their application in the removal of dyes are discussed. Moreover, the possible mechanisms for the interaction of chitosan and chitosan-based adsorbents with dyes molecules were evaluated. Finally, future prospects of using chitosan as an adsorbent for the removal of dye molecules are directed.

Simultaneously removal of Cr(VI) and Cd(II) from water using a flower-like primary battery nanosystem

In this study, a new flower-like primary battery nanosystem termed "Zn/CCP/ZIF-8" was prepared by depositing conductive carbon paint (CCP) and zeolitic imidazolate framework-8 (ZIF-8) on a zinc plate (Zn). Therein, CCP had good conductivity performance and adhesiveness, ZIF-8 and Zn/CCP/ZIF-8 possessed BET specific surface areas of 1909.5 and 1265.4 m²/g respectively. The results showed that the Zn/CCP/ZIF-8 nanosystem could effectively simultaneously adsorb hexavalent chromium (Cr(VI)) and bivalent cadmium (Cd(II)) from water. The system could promote the transfer of electrons from Zn to Cr(VI) and Cd(II) which were effectively reduced to trivalent chromium (Cr(III)) and cadmium (Cd), respectively. The resultant Zn/CCP/ZIF-8/Cr/Cd composite was then easily separated from water. The adsorption isotherm, kinetics, and thermodynamics of the prepared Zn/CCP/ZIF-8 for Cr(VI) and Cd(II) were investigated. An electrochemistry performance test proved that the Zn/CCP/ZIF-8 system was a primary battery. Notably, the Zn/CCP/ZIF-8 system substantially reduced the amounts of Cr(VI) and Cd(II) absorbed by zebrafish and water spinach, thus increasing food safety. The results of a rat test indicated that the Zn/CCP/ZIF-8 system possessed a high biosafety. This study provides a promising, eco-friendly, and facile method for the simultaneously treatment of Cr(VI) and Cd(II) contamination of water.

Facile synthesis of chitosan-based acid-resistant composite films for efficient selective adsorption properties towards anionic dyes

To effectively and selectively remove toxic anionic dyes which are heavily discharged and to promote them recovery, a sustainable cellulose nanofiber/chitosan (CNF/CS) composite film was elaborately designed through a facile procedure. Based on the strong supporting effect of CNF and excellent compatibility between CNF and CS, the composite film presents low swelling and acid-proof properties, which can prevent the adsorption process from the disintegration of adsorbent. Moreover, the positive electrical property of CNF/CS film increases the discrepancy in adsorption capacities for anionic and cationic dyes. The maximum adsorption capacity of anionic methyl orange (MO) on CNF/CS film reaches 655.23 mg/g with a desirable recyclability. The adsorption behavior attributed to a physico-chemical and monolayer adsorption process. This work opens a new route for the development of eco-friendly and highly efficient adsorbents on selective removal and recycling of anionic dyes from wastewater.

Complex interior and surface modified alginate reinforced reduced graphene oxide-hydroxyapatite hybrids: Removal of toxic azo dyes from the aqueous solution

In this study, alginate reinforced reduced graphene oxide@hydroxyapatite (rGO@HAP-Alg) hybrids have been fabricated via co-precipitation technique. The developed adsorbent was effectively utilized for the removal of Reactive Blue 4 (RB4), Indigo Carmine (IC) and Acid Blue 158 (AB158) azo dyes from aqueous solution, and found to have the adsorption efficiency of 45.56, 47.16 and 48.26 mg/g, respectively. The thermodynamic parameters demonstrated the endothermic and spontaneous nature of the adsorption process. The adsorption system was pH-dependent and showed maximum dye removal at pH 6-7, which was indicative of the electrostatic interactions, surface complexation and the hydrogen bonding mechanisms involved between the adsorbate and adsorbent during the adsorption process. Furthermore, the renewability studies demonstrated the reusability and stability of rGO@HAP-Alg hybrids up to five successive cycles. This study delivers a promising strategy for removing dye molecules and extends the potential application of rGO@HAP-Alg hybrids to treat practical dye contaminated water/wastewater.

Preparation of a novel citric acid-crosslinked Zn-MOF/chitosan composite and application in adsorption of chromium(VI) and methyl orange from aqueous solution

A novel citrate-crosslinked Zn-MOF/chitosan (ZnBDC/CSC) composite was successfully prepared by immobilizing Zn-MOF (ZnBDC) on citrate-crosslinked chitosan (CSC) using citric acid as a chemical bridge. ZnBDC/CSC was characterized by XRD, FT-IR, solid-state ¹³C NMR, BET and SEM. The adsorption of ZnBDC/CSC for Cr(VI) and MO from aqueous solutions were studied at pH 5.0. The adsorption conditions, such as adsorption time and initial concentration of Cr(VI) and MO solutions were investigated. The results indicated that ZnBDC/CSC showed high adsorption capacity for both Cr(VI) (225 ± 4 mg g^-1) and MO (202 ± 3 mg g^-1), respectively. The adsorption of Cr(VI) on ZnBDC/CSC could be well described by Langmuir isotherm model, while MO followed Freundlich model. The adsorption kinetic of Cr(VI) and MO demonstrated a better fitness to the pseudo-second order kinetic model. Thermodynamic parameters (enthalpy (ΔH), entropy (ΔS) and Gibbs free energy (ΔG)) demonstrated that the adsorption processes of Cr(VI) and MO on ZnBDC/CSC were exothermic, disordered and spontaneous at 298-318 K. The adsorption mechanism of ZnBDC/CSC for Cr(VI) could be mainly explained by electrostatic attraction and cation-π interaction, while for MO, it could be assigned to n-π and π-π interactions, electrostatic attraction and hydrogen bonding. ZnBDC/CSC could be recycled and reused for the removal of Cr(VI) and MO.

Green microwave synthesis of functionalized chitosan with robust adsorption capacities for Cr(VI) and/or RHB in complex aqueous solutions

Dinitrosalicylic acid-functionalized chitosan, CHN-DNSA, was developed and improved the adsorption property against chromium Cr(VI) and/or Rhodamine B (RHB). Here, the disposal of wastewater bearing Cr(VI) and RHB from a complex system was ascribed to significant differences in physicochemical properties. The constructed CHN-DNSA surface charge is responsible for different interactions enabling simultaneous capture of pollutants. The excellent adsorption potency of Cr(VI) at pH 3.0 was 98.4% within a remarkable 1 h and the adsorption performance was 91.1% for RHB. The ionic strength was affected, reducing the removal % of Cr(VI) to 36.7% whereas 0.1 M NaCl meliorated the removal efficiency from 91.6 to 96.2% for RHB and from 82.3 to 89.1% for a binary system. Also, the exploited elimination process of Cr(VI) and/or RHB obeyed the 2nd model of kinetics and the Freundlich system. Good recovery, superior capacity, and synthetic approach make this protocol promising for wastewater treatment.

Capability of Bacillus Subtilis to remove Pb2+ via producing lipopeptides

Lead contamination is widely found in soil and waters, which makes great threat to animal and human health. Environmentally friendly, efficient, and economical methods for the removal of Pb²⁺ pose significant challenges for environmental protection. Bacillus subtilis lipopeptide was firstly used to remove Pb²⁺ from water. In mechanisms, the lipopeptides formed complexes and chelated with Pb²⁺ via OH, CO, OCO, and NH. In kinetics, the Pb²⁺ removal process closely followed a pseudo-first-order model, and the equilibrium Pb²⁺ adsorption capacity ranged from 112.6 to 113.7 mg/g within a temperature range of 293.13-313.13 K. The Pb²⁺ removal process could be well described by a Langmuir isotherm. The maximum Pb²⁺ removal capability of lipopeptides was 164.4 mg/g in manually metal contaminated water and 130.4 mg/g in actual wastewater. Furthermore, the lipopeptides can not only decrease the amount of lead in oats grown, but also promote oat growth under Pb²⁺ stress. The results showed that lipopeptides can be used as a highly efficient adsorbent to remove Pb²⁺ from water, which means the great potential of lipopeptides in practical environments.

Fabrication of novel amine-functionalized magnetic silica nanoparticles for toxic metals: kinetic and isotherm modeling

In this research, an amine-functionalized magnetic silica nanosorbent was prepared using the co-precipitation technique, and this nanosorbent can be effortlessly detached using an external magnetic field. FTIR and SEM analyses identified that the nanosorbent holds extraordinary adsorption characteristics for toxic metals' (copper, cadmium, zinc, and nickel) removal. The adsorption-affecting parameters were optimized, and the thermodynamic studies assessed that the adsorption process seemed to be spontaneous, feasible, and exothermic. The pseudo-first-order and Freundlich models perfectly fit the kinetic and equilibrium data, respectively. Langmuir monolayer capacity of the nanosorbent was analyzed using nonlinear evaluation methods such as 419.9 mg/g for copper, 321.9 mg/g for nickel, 217.3 mg/g for cadmium, and 137.6 mg/g for zinc. The used adsorbent was simply rejuvenated using the 0.2 N HCl solution subsequently with intense agitation. The result of the present research confirms that the produced nanosorbent can be effectively utilized for industrial wastewater management.

Elaboration of novel polyaniline@Almond shell biocomposite for effective removal of hexavalent chromium ions and Orange G dye from aqueous solutions

A novel polyaniline@Almond shell (PANI@AS) biocomposite was synthesized via facile in situ chemical polymerization method. The as-synthesized adsorbent was characterized using various analytical techniques such as Fourier transform infrared spectroscopy (FTIR), X-ray energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), and potentiometric titration. A batch adsorption system was applied with the aim of investigating as-synthesized adsorbent ability to remove Cr(VI) ions and Orange G (OG) textile dye from aqueous solutions. Obtained results revealed that adsorption process was strongly depended upon the physicochemical parameters. The adsorption of Cr(VI) and OG dye onto PANI@AS was better described by the pseudo second-order-kinetic model and followed the Freundlich isotherm model. The maximum uptakes were 335.25 for Cr(VI) and 190.98 mg g^-1 for OG dye. We further evaluated that PANI@AS biocomposite could be regenerated easily with NaOH solution and efficiently reused for Cr(VI) and OG dye removal from aqueous media. Thus, these results indicated the potential practical application of PANI@AS biocomposite for wastewater treatment.

Chitosan-reduced graphene oxide composites with 3D structures as effective reverse dispersed solid phase extraction adsorbents for pesticides analysis

Chitosan-reduced graphene oxide composites with 3D structures (3D CS-rGO) were prepared via a facile solvothermal synthetic strategy. The reduction of GO and crosslinking of rGO nanosheets with CS into 3D structures were achieved simultaneously in one step. The resulting 3D CS-rGO composite achieves high adsorption capacities of catechins, caffeine and pigments from tea acetonitrile extraction, especially for catechins, which is 10 times higher than that of GO (179.3 mg g^-1vs. 18.7 mg g^-1). The 3D CS-rGO composite exhibits the best removal efficiency of matrix interference in comparison to other traditional adsorbents, which could diminish the matrix effect on targeted pesticides by 1%-55%. A determination method of 70 kinds of pesticides is successfully established for tea based on 3D CS-rGO composite as an efficient reverse dispersed solid phase extraction adsorbent. The established method has the great advantages of operation simplicity, being time saving and high purification performance using only one kind and a low amount of adsorbent. 3D graphene based materials are expected to be promising adsorbents for sample pretreatment in trace contaminant analysis of complex food samples.

Preparation and characterization of poly aniline modified chitosan embedded with ZnO-Fe3O4 for Cu(II) removal from aqueous solution

Poly aniline modified chitosan embedded with ZnO/Fe₃O₄ nanocomposites were synthesized using a precipitation method and applied to the removal of Cu(II) from aqueous solution. The synthesized nanocomposite was characterized by FT-IR, XRD, FESEM, TEM, EDS, TGA, BET and zeta-potential analyses. The adsorption batch experiments were conducted as a function of five effective parameters including pH, contact time, initial concentration of copper, temperature, and adsorbent dosage using a central composite design (CCD) in response surface methodology (RSM). Contour and surface plots were used to determine the interaction effects of main factors and optimum conditions of process. The regression equation coefficients were calculated and the data confirmed the validity of second-order polynomial equation for the removal of Cu(II) with novel absorbent. Analysis of variance (ANOVA) showed a high coefficient of determination value (R²) for copper removal being 0.99. The optimum level of the pH, temperature, initial concentration of copper, adsorbent dosage and contact time for maximum Cu(II) removal (94.51%) were found to be 6.5, 31 °C, 82 mg L^-1, 0.81 g L^-1, and 51 min, respectively. It was confirmed from XPS and EDS analyses that heavy metal ions were present on the surface of nanocomposite after adsorption. The adsorption equilibrium data fitted well with the Langmuir isotherm model and the adsorption process followed the pseudo-second-order and intra-particle diffusion kinetic model. The saturated adsorption capacity is found to be 328.4 mg/g. Thermodynamics analysis suggests that the adsorption process is endothermic, with increasing entropy and spontaneous in nature. Further recycling experiments show that nanocomposite still retains 95% of the original adsorption following the 5th adsorption-desorption cycle. The effects of coexist cation ions on the adsorption of Cu(II) was also investigated under optimal condition. All the results demonstrate that nanocomposite is a potential recyclable adsorbent for hazardous metal ions in wastewater.

Synthesis and Characterization of Acetic Acid-Doped Polyaniline and Polyaniline⁻Chitosan Composite

Polyaniline–chitosan (PAni–Cs) composite films were synthesized using a solution casting method with varying PAni concentrations. Polyaniline powders used in the composite synthesis were polymerized using acetic acid as the dopant media. Raman spectroscopy revealed that the PAni powders synthesized using hydrochloric acid and acetic acid did not exhibit significant difference to the chemical features of PAni, implying that PAni was formed in varying concentrations of the dopant media. The presence of agglomerated particles on the surface of the Cs composite, which may have been due to the presence of PAni powders, was observed with scanning electron microscope–energy dispersive X-ray spectroscopy (SEM–EDX). Ultraviolet–visible (UV–Vis) spectroscopy further showed the interaction of PAni with Cs where the Cs characteristic peak shifted to a higher wavelength. Cell viability assay also revealed that the synthesized PAni–Cs composites were nontoxic and may be utilized for future biomedical applications.