Effective Adsorption and Sensitive Detection of Cr(VI) by Chitosan/Cellulose Nanocrystals Grafted with Carbon Dots Composite Hydrogel

Recent advances in nanocellulose based hydrogels: Preparation strategy, typical properties and food application

Nanocellulose has been favored as one of the most promising sustainable nanomaterials, due to its competitive advantages and superior performances such as hydrophilicity, renewability, biodegradability, biocompatibility, tunable surface features, excellent mechanical strength, and high specific surface area. Based on the above properties of nanocellulose and the advantages of hydrogels such as high water absorption, adsorption, porosity and structural adjustability, nanocellulose based hydrogels integrating the benefits of both have attracted extensive attention as promising materials in various fields. In this review, the main fabrication strategies of nanocellulose based hydrogels are initially discussed in terms of different crosslinking methods. Then, the typical properties of nanocellulose based hydrogels are comprehensively summarized, including porous structure, swelling ability, adsorption, mechanical, self-healing, smart response performances. Especially, relying on these properties, the general application of nanocellulose based hydrogels in food field is also discussed, mainly including food packaging, food detection, nutrient embedding delivery, 3D food printing, and enzyme immobilization. Finally, the safety of nanocellulose based hydrogel is summarized, and the current challenges and future perspectives of nanocellulose based hydrogels are put forward.

Review of carbon dot-hydrogel composite material as a future water-environmental regulator

As water pollution and scarcity pose severe threats to the sustainable progress of human society, it is important to develop a method or materials that can accurately and efficiently detect pollutants and purify aquatic environments or exploit marine resources. The compositing of photoluminescent and hydrophilic carbon dots (CDs) with hydrogels bearing three-dimensional networks to form CD-hydrogel composites to protect aquatic environments is a "win-win" strategy. Herein, the feasibility of the aforementioned method has been demonstrated. This paper reviews the recent progress of CD-hydrogel materials used in aquatic environments. First, the synthesis methods for these composites are discussed, and then, the composites are categorized according to different methods of combining the raw materials. Thereafter, the progress in research on CD-hydrogel materials in the field of water quality detection and purification is reviewed in terms of the application of the mechanisms. Finally, the current challenges and prospects of CD-hydrogel materials are described. These results are expected to provide insights into the development of CD-hydrogel composites for researchers in this field.

Phosphorylated chitin from shrimp shell waste: A robust solution for cadmium remediation

In this work, chitin (CT) was isolated from shrimp shell waste (SSW) and was then phosphorylated using diammonium hydrogen phosphate (DAP) as a phosphorylating agent in the presence of urea. The prepared samples were characterized using Scanning Electron Microscopy (SEM) and EDX-element mapping, Fourier Transform Infrared Spectroscopy (ATR-FTIR), X-Ray Diffraction (XRD), Thermogravimetric Analysis (TGA/DTG), conductometric titration, Degree of Substitution (DS) and contact angle measurements. The results of characterization techniques reveal the successful extraction and phosphorylation of chitin. The charge content of the phosphorylated chitin (P-CT) was 1.510 mmol·kg-1, the degree of substitution of phosphorus groups grafted on the CT surface achieved the value of 0.33. The adsorption mechanisms appeared to involve electrostatic attachment, specific adsorption (CdO or hydroxyl binding), and ion exchange. Regarding the adsorption of Cd2+, the effect of the adsorbent mass, initial concentration of Cd2+, contact time, pH, and temperature were studied in batch experiments, and optimum values for each parameter were identified. The experimental results revealed that P-CT enhanced the Cd2+ removal capacity by 17.5 %. The kinetic analyses favored the pseudo-second-order model over the pseudo-first-order model for describing the adsorption process accurately. Langmuir model aptly represented the adsorption isotherms, suggesting unimolecular layer adsorption with a maximum capacity of 62.71 mg·g-1 under optimal conditions of 30 °C, 120 min, pH 8, and a P-CT dose of 3 g·L-1. Regeneration experiments evidenced that P-CT can be used for 6 cycles without significant removal capacity loss. Consequently, P-CT presents an efficient and cost-effective potential biosorbent for Cd2+ removal in wastewater treatment applications.

Fabrication of a porous Urea@MIL-100(Fe)/CI-MCC/SA hydrogel for All-In-One adsorption, removal and fluorescence monitoring of nitrite

In this paper, a novel All-In-One Urea@MIL-100(Fe)/CI-MCC/SA hydrogel platform was generated by microcrystalline cellulose (MCC) functionalized with pH-response probe (CI), MIL-100 (Fe) and sodium alginate (SA), which was as a carrier of urea to adsorb, remove and monitor NO2-. Under acidic condition, the fluorescent hydrogel platform could produce N2, CO2 and H2O through the diazotization and redox reaction between urea and NO2- with a removal efficiency up to 99.8%, and could also character a good adsorption property for NO2- due to the positive charges of protonation (the maximum adsorption capacity was 21.67 mg g-1), and the adsorption kinetics conformed to pseudo-second-order model. By carried out the NO2- removal step in fluorescent hydrogel platform, NO2- could also be detected indirectly by sensing the changes of pH within 15 min. The linear response range was 0-0.005 M, and the detection limit (LOD) was 74 μM. These results demonstrated that this All-In-One Urea@MIL-100(Fe)/CI-MCC/SA hydrogel platform had great potential in environment. This strategy for the removal and monitoring of NO2- could be employed to related applications in water purification and environmental protection. ENVIRONMENTAL IMPLICATION: Nitrite is one of the important indicators of water monitoring, which is harmful to human and environment. The removal and monitoring of nitrite in industrial wastewater and surface water is very important, but there are no studies about it at present. Based on the fact that urea can react with nitrite to produce green products, we synthesized a novel functional hydrogel to achieve adsorption, removal and fluorescence monitoring of nitrite for the first time. Besides, the practicability of the material in environmental water samples was verified through the detection of nitrite in simulated wastewater.

Gum acacia based hydrogels and their composite for waste water treatment: A review

The non-toxic nature of natural polysaccharides and their biodegradability makes them the first choice of researchers. Various natural polysaccharides are available nowadays, like cellulose, starch, chitosan, gum acacia, guar gum etc. Among these, gum acacia is a common natural polysaccharide presently used in research and technology. It is highly biodegradable, pH stable and shows appropriate water solubility. It is used in research to synthesize hydrogels and hydrogel nanocomposites for various applications because of its antimicrobial, anti-inflammatory and excellent absorption properties. The major fields of applications include the stabilization of metal nanoparticles in the form of nanocomposites, wound dressing materials, delivery systems of various drugs and pharmaceutical agents, bioengineering, tissue engineering, purification of water, synthesis of antibacterial and antifungal composites for agricultural improvements, and many others. Due to the increasing problem of water pollution, the major focus is on research helping to reduce this problem. Gum acacia-based hydrogel and hydrogel composites were synthesized and tested for pollutant removal efficiency from wastewater by different researchers. The research on gum acacia hydrogel and their hydrogel composite applications for water purification, as well as their synthesis processes and properties, are summarized in this review article.

Efficient Removal of Heavy Metals from Aqueous Solution Using Licorice Residue-Based Hydrogel Adsorbent

The removal of heavy metals through adsorption represents a highly promising method. This study focuses on the utilization of an abundant cellulose-rich solid waste, licorice residue (LR), as a natural material for hydrogel synthesis. To this end, LR-EPI hydrogels, namely, LR-EPI-5, LR-EPI-6 and LR-EPI-8, were developed by crosslinking LR with epichlorohydrin (EPI), specifically targeting the removal of Pb, Cu, and Cr from aqueous solutions. Thorough characterizations employing Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscopy confirmed the successful crosslinking of LR-EPIs by EPI, resulting in the formation of porous and loosely structured hydrogels. Batch studies demonstrated the high efficacy of LR-EPI hydrogels in removing the three heavy metal ions from aqueous solutions. Notably, LR-EPI-8 exhibited the highest adsorption capacity, with maximum capacities of 591.8 mg/g, 458.3 mg/g, and 121.4 mg/g for Pb²⁺, Cr³⁺, and Cu²⁺, respectively. The adsorption processes for Pb²⁺ and Cu²⁺ were well described by pseudo-second-order kinetics and the Langmuir model. The adsorption mechanism of LR-EPI-8 onto heavy metal ions was found to involve a combination of ion-exchange and electrostatic interactions, as inferred from the results obtained through X-ray photoelectron spectroscopy and FTIR. This research establishes LR-EPI-8 as a promising adsorbent for the effective removal of heavy metal ions from aqueous solutions, offering an eco-friendly approach for heavy metal removal and providing an environmentally sustainable method for the reutilization of Chinese herb residues. It contributes to the goal of "from waste, treats waste" while also addressing the broader need for heavy metal remediation.

Surface engineered functional biomaterials for hazardous pollutants removal from aqueous environment

The issue of water contamination by heavy metal ions as highly persistent pollutants with harmful influence primarily on biological systems, even in trace levels, has become a great environmental concern globally. Therefore, there is a need for the use of highly sensitive techniques or preconcentration methods for the removal of heavy metal ions at trace levels. Thus, this research investigates a novel approach by examining the possibility of using pomegranate (Punica granatum) peel layered material for the simultaneous preconcentration of seven heavy metal ions; Cd(II), Co(II), Cr(III), Cu(II), Mn(II), Ni(II) and Pb(II) from aqueous solution and three river water samples. The quantification of the heavy metals was performed by the means of FAAS technique. The characterization of biomaterial was performed by SEM/EDS, FTIR analysis and pHpzc determination before and after the remediation process. The reusability study, as well as the influence of interfering ions (Ca, K, Mg, Na and Zn) were evaluated. The conditions of preconcentration by the column method included the optimization of solution pH (5); flow rate (1.5 mL/min), a dose of biosorbent (200 mg), type of the eluent (1 mol/L HNO₃), sample volume (100 mL) and sorbent fraction (<0.25 mm). The biosorbent capacity ranged from 4.45 to 57.70 μmol/g for the investigated heavy metals. The practical relevance of this study is further extended by novel data regarding adsorbent cost analysis (17.49 $/mol). The Punica granatum sorbent represents a highly effective and economical biosorbent for the preconcentration of heavy metal ions for possible application in industrial sectors.

Cost estimation of synthesis and utilization of nano-adsorbents on the laboratory and industrial scales: A detailed review

The recent regulations pertaining to the circular economy have unlocked new prospects for researchers. In contrast to the unsustainable models associated with the linear economy, integration of concepts of circular economy braces reducing, reusing, and recycling of waste materials into high-end products. In this regard, adsorption is a promising and cost-effective water treatment technology for handling conventional and emerging pollutants. Numerous studies are published annually to investigate the technical performance of nano-adsorbents and nanocomposites in terms of adsorption capacity and kinetics. Yet, economic performance evaluation is rarely discussed in the literature. Even if an adsorbent shows high removal efficiency towards a specific pollutant, its high preparation and/or utilization costs might hinder its real-life use. This tutorial review aims at illustrating cost estimation methods for the synthesis and utilization of conventional and nano-adsorbents. The current treatise discusses the synthesis of adsorbents on a laboratory scale where the raw material, transportation, chemical, energy, and any other costs are discussed. Moreover, equations for estimating the costs at the large-scale adsorption units for wastewater treatment are illustrated. This review focuses on introducing these topics to non-specialized readers in a detailed but simplified manner.

Preparation and modification of nanocellulose and its application to heavy metal adsorption: A review

Heavy metals are a notable pollutant in aquatic ecosystems that results in many deadly diseases of the human body after enrichment through the food chain. As an environmentally friendly renewable resource, nanocellulose can be competitive with other materials at removing heavy metal ions due to its large specific surface area, high mechanical strength, biocompatibility and low cost. In this review, the research status of modified nanocellulose for heavy metal adsorbents is primarily reviewed. Two primary forms of nanocellulose are cellulose nanocrystals (CNCs) and cellulose nanofibers (CNFs). The preparation process of nanocellulose was derived from natural plants, and the preparation process included noncellulosic constituent removal and extraction of nanocellulose. Focusing on heavy metal adsorption, the modification of nanocellulose was explored in depth, including direct modification methods, surface grafting modification methods based on free radical polymerization and physical activation. The adsorption principles of nanocellulose-based adsorbents when removing heavy metals are analyzed in detail. This review may further facilitate the application of the modified nanocellulose in the field of heavy metal removal.

Ecofriendly adsorption and sensitive detection of Hg (II) by biomass-derived nitrogen-doped carbon dots: process modelling using central composite design

In this study, luminescent bio-adsorbent nitrogen-doped carbon dots (N-CDs) was produced and applied for the removal and detection of Hg (II) from aqueous media. N-CDs were synthesized from oil palm empty fruit bunch carboxymethylcellulose (CMC) and urea. According to several analytical techniques used, the obtained N-CDs display graphitic core with an average size of 4.2 nm, are enriched with active sites, stable over a wide range of pH and have great resistance to photobleaching. The N-CDs have bright blue emission with an improved quantum yield (QY) of up to 35.5%. The effect of the variables including pH, adsorbent mass, initial concentration and incubation time on the removal of Hg (II) was investigated using central composite design. The statistical results confirmed that the adsorption process could reach equilibrium within 30 min. The reduced cubic model (R² = 0.9989) revealed a good correlation between the observed values and predicted data. The optimal variables were pH of 7, dose of 0.1 g, initial concentration of 100 mg/L and duration of 30 min. Under these conditions, adsorption efficiency of 84.6% was obtained. The adsorption kinetic data could be well expressed by pseudo-second-order kinetic and Langmuir isotherm models. The optimal adsorption capacity was 116.3 mg g^-1. Furthermore, the adsorbent has a good selectivity towards Hg (II) with a detection limit of 0.01 μM due to the special interaction between Hg (II) and carboxyl/amino groups on the edge of N-CDs. This work provided an alternative direction for constructing low-cost adsorbents with effective sorption and sensing of Hg (II).

Hexavalent Chromium Removal from Industrial Wastewater by Adsorption and Reduction onto Cationic Cellulose Nanocrystals

Cationic cellulose nanocrystals (CCNC) are lignocellulosic bio-nanomaterials that present large, specific areas rich with active surface cationic groups. This study shows the adsorption removal of hexavalent chromium (Cr(VI)) from industrial wastewaters by the CCNC. The CCNC were synthetized through periodate oxidation and Girard's reagent-T cationization. The high value of CCNCs cationic groups and anionic demand reveal probable nanocrystal-Cr(VI) attraction. Adsorption was performed with synthetic Cr(VI) water at different pH, dosage, Cr(VI) concentration and temperature. Fast removal of Cr(VI) was found while operating at pH 3 and 100 mg·L^-1 of dosage. Nevertheless, a first slower complete removal of chromium was achieved by a lower CCNC dosage (40 mg·L^-1). Cr(VI) was fully converted by CCNC into less-toxic trivalent species, kept mainly attached to the material surface. The maximum adsorption capacity was 44 mg·g^-1. Two mechanisms were found for low chromium concentrations (Pseudo-first and pseudo-second kinetic models and continuous growth multi-step intraparticle) and for high concentrations (Elovich model and sequential fast growth-plateau-slow growth intraparticle steps). The Sips model was the best-fitting isotherm. Isotherm thermodynamic analysis indicated a dominant physical sorption. The Arrhenius equation revealed an activation energy between physical and chemical adsorption. CCNC application at selected conditions in industrial wastewater achieved a legal discharge limit of 40 min.

Biosorption of Cr (VI) by acid-modified based-waste fungal biomass from Calocybe indica fruiting bodies production

The world is going through a colossal drinking water scarcity. Unchecked discharge (even at trace levels) of Cr (VI) from industries into water bodies is a serious environmental concern. Here, we report waste fungal biomass (WFB) for the detoxification and removal of chromium ions. Biomass understudy was collected from Calocybe indica fruiting bodies. WFB was used after drying and pretreatment with two distinctive chemical methods, which improved the remediation effectiveness of Cr (VI). Light microscope and Field emission Scanning microscope (FESEM) were employed to elucidate the surface morphology of waste fungal biomass. While Fourier-Transform Infrared-Spectroscopy (FTIR) and Energy Dispersive X-Ray analysis (EDAX) were deployed to explore the mechanism of interaction between Cr (VI) anion and waste fungal biomass. X-ray Photoelectron Spectroscopy (XPS) analyses demonstrated considerable conversion of Cr (VI) into nontoxic Cr (III) species. The most favorable condition for optimum Cr (VI) remediation of 99.66% by treated waste fungal biomass (TWFB) occurred at pH 3, contact time 10 min, adsorbent dosage 3 gL^-1, Cr (VI) concentration 4 mgL^-1, stirring speed 140 rpm, and temperature 320 K, where for untreated waste fungal biomass (UWFB) the optimum of 85% remediation occurred at a contact time 15 min, and adsorbent dosage 2 gL^-1 whereas other experimental conditions remained identical as TWFB biosorbent. Pseudo-second-order kinetics (R² > 0.99) model matched the adsorption rate. And, the Freundlich isotherm model (R² > 0.99) is shown to be a better match for the experimental data. The optimum amount of Cr (VI) adsorbed by the TWFB and UWFB were 205.8 ± 10.1 and 72.85 ± 2.36 mgg^-1, respectively. Thermodynamic parameters revealed that the adsorption was spontaneous (ΔG ˂ 0), endothermic (ΔH > 0), and entropy-driven (ΔS > 0). The generated WFB adsorbent also has significant recycling potential. After five cycles of regeneration and adsorption. It can still keep up good remediation effectiveness of Cr (VI) ions to 85.5.

Removal of Cr(VI) from Wastewater Using Graphene Oxide Chitosan Microspheres Modified with α-FeO(OH)

Graphene oxide and chitosan microspheres modified with α−FeO(OH) (α−FeO(OH)/GOCS) are prepared and utilized to investigate the performance and mechanism for Cr(VI) removal from aqueous solutions and the possibility of Fe secondary pollution. Batch experiments were carried out to identify the effects of pH, mass, and volume ratio (m/v), coexisting ions, time (t), temperature (T), and Cr(VI) initial concentration (C0) on Cr(VI) removal, and to evaluate adsorption kinetics, equilibrium isotherm, and thermodynamics, as well as the possibility of Fe secondary pollution. The results showed that Cr(VI) adsorption increased with C0, t, and T but decreased with increasing pH and m/v. Coexisting ions inhibited Cr(VI) adsorption, and this inhibition increased with increasing concentration. The influence degrees of anions and cations on the Cr(VI) adsorption in descending order were SO42− > PO42− > NO3− > Cl− and Ca2+ > Mg2+ > Mn2+, respectively. The equilibrium adsorption capacity of Cr(VI) was the highest at 24.16 mg/g, and the removal rate was 97.69% under pH = 3, m/v = 1.0 g/L, T = 298.15 K, and C0 = 25 mg/L. Cr(VI) adsorption was well fitted to a pseudo-second-order kinetic model and was spontaneous and endothermic. The best fit of Cr(VI) adsorption with the Langmuir and Sips models indicated that it was a monolayer and heterogeneous adsorption. The fitted maximum adsorption capacity was 63.19 mg/g using the Sips model under 308.15 K. Cr(VI) removal mainly included electrostatic attraction between Cr(VI) oxyanions with surface Fe−OH2+, and the adsorbed Cr(VI) was partially reduced to Cr(III) and then precipitated on the surface. In addition, there was no Fe secondary pollution during Cr(VI) adsorption.

Recent Advances in Synthesis, Modification, Characterization, and Applications of Carbon Dots

Although there is significant progress in the research of carbon dots (CDs), some challenges such as difficulty in large-scale synthesis, complicated purification, low quantum yield, ambiguity in structure-property correlation, electronic structures, and photophysics are still major obstacles that hinder the commercial use of CDs. Recent advances in synthesis, modification, characterization, and applications of CDs are summarized in this review. We illustrate some examples to correlate process parameters, structures, compositions, properties, and performances of CDs-based materials. The advances in the synthesis approach, purification methods, and modification/doping methods for the synthesis of CDs are also presented. Moreover, some examples of the kilogram-scale fabrication of CDs are given. The properties and performance of CDs can be tuned by some synthesis parameters, such as the incubation time and precursor ratio, the laser pulse width, and the average molar mass of the polymeric precursor. Surface passivation also has a significant influence on the particle sizes of CDs. Moreover, some factors affect the properties and performance of CDs, such as the polarity-sensitive fluorescence effect and concentration-dependent multicolor luminescence, together with the size and surface states of CDs. The synchrotron near-edge X-ray absorption fine structure (NEXAFS) test has been proved to be a useful tool to explore the correlation among structural features, photophysics, and emission performance of CDs. Recent advances of CDs in bioimaging, sensing, therapy, energy, fertilizer, separation, security authentication, food packing, flame retardant, and co-catalyst for environmental remediation applications were reviewed in this article. Furthermore, the roles of CDs, doped CDs, and their composites in these applications were also demonstrated.

Preparation of Graphene Oxide/Cellulose Composites with Microcrystalline Cellulose Acid Hydrolysis Using the Waste Acids Generated by the Hummers Method of Graphene Oxide Synthesis

The Hummers method is the most commonly used method to prepare graphene oxide (GO). However, many waste acids remain in the raw reaction mixture after the completion of this reaction. The aim of this study was to reuse these waste acids efficiently. In this study, microcrystalline cellulose (MCC) was directly dissolved in the mixture after the high-temperature reaction of the Hummers method. The residual acid was used to hydrolyze MCC, and the graphene oxide/microcrystalline cellulose (GO/MCC) composites were prepared, while the acid was reused. The effects of MCC addition (0.5 g, 1.0 g, and 1.5 g in 20 mL) on the properties of the composites were discussed. The structure, composition, thermal stability, and hydrophobicity of GO/MCC composites were characterized and tested by SEM, XRD, FTIR, TG, and contact angle tests. The results showed that MCC could be acid hydrolyzed into micron and nano-scale cellulose by using the strong acidity of waste liquid after GO preparation, and it interacted with the prepared GO to form GO/MCC composites. When the addition amount of MCC was 1 g, the thermal stability of the composite was the highest due to the interaction between acid-hydrolyzed MCC and GO sheets. At the same time, the hydrophobic property of the GO/MCC composite is better than that of the GO film. The freeze-dried GO/MCC composites are more easily dispersed in water and have stronger stability.