Bacterial cellulose aerogels: Influence of oxidation and silanization on mechanical and absorption properties

Synthesis strategies, regeneration, cost analysis, challenges and future prospects of bacterial cellulose-based aerogels for water treatment: A review

Clean water is an integral part of industries, agricultural activities and human life, but water contamination by toxic dyes, heavy metals, and oil spills is increasingly serious in the world. Aerogels with unique properties such as highly porous and extremely low density, tunable surface modification, excellent reusability, and thermal stability can contribute to addressing these issues. Thanks to high purity, biocompatibility and biodegradability, bacterial cellulose can be an ideal precursor source to produce aerogels. Here, we review the modification, regeneration, and applications of bacterial cellulose-based aerogels for water treatment. The modification of bacterial cellulose-based aerogels undergoes coating of hydrophobic agents, carbonization, and incorporation with other materials, e.g., ZIF-67, graphene oxide, nanoparticles, polyaniline. We emphasized features of modified aerogels on porosity, hydrophobicity, density, surface chemistry, and regeneration. Although major limits are relevant to the use of toxic coating agents, difficulty in bacterial culture, and production cost, the bacterial cellulose aerogels can obtain high performance for water treatment, particularly, catastrophic oil spills.

Aerogels based on Bacterial Nanocellulose and their Applications

Microbial cellulose stands out for its exceptional characteristics in the form of biofilms formed by highly interlocked fibrils, namely, bacterial nanocellulose (BNC). Concurrently, bio-based aerogels are finding uses in innovative materials owing to their lightweight, high surface area, physical, mechanical, and thermal properties. In particular, bio-based aerogels based on BNC offer significant opportunities as alternatives to synthetic or mineral counterparts. BNC aerogels are proposed for diverse applications, ranging from sensors to medical devices, as well as thermal and electroactive systems. Due to the fibrous nanostructure of BNC and the micro-porosity of BNC aerogels, these materials enable the creation of tailored and specialized designs. Herein, a comprehensive review of BNC-based aerogels, their attributes, hierarchical, and multiscale features are provided. Their potential across various disciplines is highlighted, emphasizing their biocompatibility and suitability for physical and chemical modification. BNC aerogels are shown as feasible options to advance material science and foster sustainable solutions through biotechnology.

Eco-friendly bacterial cellulose/MXene aerogel with excellent photothermal and electrothermal conversion capabilities for efficient separation of crude oil/seawater mixture

Developing novel absorbent materials targeting high-efficiency, low-energy-consumption, and environmental-friendly oil spill cleanup is still a global issue. Porous absorbents endowed with self-heating function are an attractive option because of that they are able to in-situ heat crude oil and dramatically reduce oil viscosity for efficient remediation. Herein, we facilely prepared an eco-friendly multifunctional bacterial cellulose/MXene aerogel (P-SBC/MXene aerogel) for rapid oil recovery. Thanks to excellent full solar spectrum absorption (average absorbance = 96.6 %), efficient photo-thermal conversion, and superior electrical conductivity (electrical resistance = 36 Ω), P-SBC/MXene aerogel exhibited outstanding photothermal and electrothermal capabilities. Its surface temperature could quickly reach 93 °C under 1.0 kW/m2 solar irradiation and 124 °C under 3.0 V voltage respectively, enabling effective heat transfer toward spilled oil. The produced heat significantly decreased crude oil viscosity, allowing P-SBC/MXene aerogel to rapidly absorb oil. By combining solar heating and Joule heating, P-SBC/MXene aerogel connected to a pump-assisted absorption device was capable of achieving all-weather crude oil removal from seawater (crude oil flux = 630 kg m-2 h-1). More notably, P-SBC/MXene aerogel showed splendid outdoor crude oil separation performance. Based on remarkable crude oil/seawater separation ability, the versatile aerogel provides a promising way to deal with large-area oil spills.

Development of silanized bacterial cellulose aerogels for the incorporation of natural oils with healing properties: Copaiba (Copaifera officinalis), bourbon geranium (Pelargonium X ssp.) essential oils and buriti (Mauritia flexuosa) vegetable oil

Bacterial cellulose (BC) represents a promising biomaterial, due to its unique and versatile properties. We report, herein, on purposely-designed structural modifications of BC that enhance its application as a wound dressing material. Chemical modification of the functional groups of BC was performed initially to introduce a hydrophobic/oleophilic character to its surface. Specifically, silanization was carried out in an aqueous medium using methyltrimethoxisilane (MTMS) as the silanizing agent, and aerogels were subsequently prepared by freeze-drying. The BC-MTMS aerogel obtained displayed a highly porous (99 %) and lightweight structure with an oil absorption capacity of up to 52 times its dry weight. The XRD pattern indicated that the characteristic crystallographic planes of the native BC were maintained after the silanization process. Thermal analysis showed that the thermal stability of the BC-MTMS aerogel increased, as compared to the pure BC aerogel (pBC). Moreover, the BC-MTMS aerogel was not cytotoxic to fibroblasts and keratinocytes. In the second step of the study, the incorporation of natural oils into the aerogel's matrix was found to endow antimicrobial and/or healing properties to BC-MTMS. Bourbon geranium (Pelargonium X ssp.) essential oil (GEO) was the only oil that exhibited antimicrobial activity against the tested microorganisms, whereas buriti (Mauritia flexuosa) vegetable oil (BVO) was non-cytotoxic to the cells. This study demonstrates that the characteristics of the BC structure can be modified, while preserving its intrinsic features, offering new possibilities for the development of BC-derived materials for specific applications in the biomedical field.

Asphaltene-Derived Graphene Quantum Dots for Controllable Coatings on Glass, Fabrics, and Aerogels

Graphene quantum dots (GQDs) derived from natural asphaltene byproducts can produce controlled hydrophobic or hydrophilic interfaces on glass, fabrics, and aerogels. A set of facile solvent extraction methods were used to isolate and chemically prepare materials with different surface functionalities from a commercially derived asphaltene precursor. The organic-soluble fraction was used to create hydrophobic and water-repellent surfaces on glass and cotton fabrics. The GQD solutions could also penetrate the pores of a silica aerogel, rendering it hydrophobic. Alternatively, by extracting the more polar fraction of the GQDs and oxidizing their surfaces, we also demonstrate strongly hydrophilic coatings. This work shows that naturally abundant GQD-containing materials can produce interfaces with the desired wettability properties through a straightforward tuning of the solvent extraction procedure. Owing to their natural abundance, low toxicity, and strong fluorescence, asphaltene-derived GQDs could thus be applied, in bulk, toward a wide range of tunable surface coatings. This approach, moreover, uses an important large-scale hydrocarbon waste material, thereby offering a sustainable alternative to the disposal of asphaltene wastes.

Cellulose Nanofibril Stabilized Pickering Emulsion Templated Aerogel with High Oil Absorption Capacity

Nanocellulose-based aerogels, featuring a three-dimensional porous structure, are considered as a desirable green absorbent because of their exceptional absorption performance as well as the abundance and renewability of the raw material. However, these aerogels often require hydrophobic modification or carbonization, which is often environmentally harmful and energy-intensive. In this study, we introduce a Pickering-emulsion-templating approach to fabricate a cellulose nanofibril (CNF) aerogel with a hierarchical pore structure, allowing for high oil absorption capacity. n-Hexane-CNF oil-in-water Pickering emulsions are prepared as an emulsion template, which is further lyophilized to create a hollow microcapsule-based CNF (HM-CNF) aerogel with a density ranging from 1.3 to 6.1 mg/cm3 and a porosity of ≥99.6%. Scanning electron microscopy and Brunauer-Emmett-Teller analyses reveal the HM-CNF aerogel's hierarchical pore structure, originating from the CNF Pickering emulsion template, and also confirm the aerogel's very high surface area of 216.6 m2/g with an average pore diameter of 8.6 nm. Furthermore, the aerogel exhibits a maximum absorption capacity of 354 g/g and 166 g/g for chloroform and n-hexadecane, respectively, without requiring any surface modification or chemical treatment. These combined findings highlight the potential of the Pickering-emulsion-templated CNF aerogel as an environmentally sustainable and high-performance oil absorbent.

Understanding Nanocellulose-Water Interactions: Turning a Detriment into an Asset

Modern technology has enabled the isolation of nanocellulose from plant-based fibers, and the current trend focuses on utilizing nanocellulose in a broad range of sustainable materials applications. Water is generally seen as a detrimental component when in contact with nanocellulose-based materials, just like it is harmful for traditional cellulosic materials such as paper or cardboard. However, water is an integral component in plants, and many applications of nanocellulose already accept the presence of water or make use of it. This review gives a comprehensive account of nanocellulose-water interactions and their repercussions in all key areas of contemporary research: fundamental physical chemistry, chemical modification of nanocellulose, materials applications, and analytical methods to map the water interactions and the effect of water on a nanocellulose matrix.

Mechanically Strong and Thermally Stable Chemical Cross-Linked Polyimide Aerogels for Thermal Insulator

High-performance thermal insulating materials are highly desirable in several fields, especially for thermal insulation of buildings to reduce energy consumption. Owing to the remarkable thermal stability, high porosity, low density, and outstanding mechanical features, polyimide (PI) aerogels have attracted great attention. In this work, chemical cross-linked PI (CCPI) aerogels were fabricated via freeze-drying and thermal imidization, which possess outstanding mechanical properties, good thermal stability, and excellent thermal insulation characteristics. The chemically cross-linked structure can effectively inhibit shrinkage, while retaining the structural integrity, resulting in the lower density and lower shrinkage of the materials. In this paper, completely imidized and highly cross-linked polyimide aerogels were synthesized by using p-phenylenediamine (PDA), 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA), and the cross-linker 2,3,6,7,14,15-hexaaminotriptycene (HMT). The CCPI aerogels with excellent properties, such as covalently cross-linked chemical structure, low density (0.069 g/cm3), low volume shrinkage (10%), high decomposition temperature (Td5% = 587 °C), and low thermal conductivity (25 mW m-1K-1) are in high demand in the field of thermal insulation. This work furnishes a new method for the development of polymer-based thermal insulation materials for various prospective applications.

Nanofibrous scaffolds based on bacterial cellulose crosslinked with oxidized sucrose

In this work, oxidized sucrose (OS), which is a safe bio-based and non-toxic polyaldehyde, was used as a crosslinker in defibrillated bacterial cellulose (BC) sponges obtained by freeze-drying. For mimicking the proteins' crosslinking, BC was first modified with an aminosilane to partially replace the OH groups on the BC surface with more reactive amino groups. Further, the aminosilane-grafted bacterial cellulose (BCA) was crosslinked with OS in different concentrations and thermally cured. Functionalized bacterial celluloses showed a good thermal stability, comparable to that of unmodified cellulose and much improved mechanical properties. A threefold increase in the compression strength was obtained for the BCA scaffold after crosslinking and curing. This was correlated with the uniform pore structure emphasized by the micro-CT and SEM analyses. The OS-crosslinked BCA scaffolds were not cytotoxic and showed a porosity of around 80 %, which was almost 100 % open porosity. This study shows that the crosslinking of aminated BC scaffolds with OS allows the obtaining of 3D cellulose structures with good mechanical properties and high porosity, suitable for soft tissue engineering. The results recommend this new method as an innovative approach to obtaining biomaterial scaffolds that mimic the natural extracellular matrix.

Cost-Effective Synthesis of Bacterial Cellulose and Its Applications in the Food and Environmental Sectors

Bacterial cellulose (BC), also termed bio-cellulose, has been recognized as a biomaterial of vital importance, thanks to its impressive structural features, diverse synthesis routes, high thermomechanical properties, and its ability to combine with multiple additives to form composites for a wide range of applications in diversified areas. Its purity, nontoxicity, and better physico-mechanical features than plant cellulose (PC) make it a better choice for biological applications. However, a major issue with the use of BC instead of PC for various applications is its high production costs, mainly caused by the use of expensive components in the chemically defined media, such as Hestrin–Schramm (HS) medium. Furthermore, the low yield of BC-producing bacteria indirectly accounts for the high cost of BC-based products. Over the last couple of decades, extensive efforts have been devoted to the exploration of low-cost carbon sources for BC production, besides identifying efficient bacterial strains as well as developing engineered strains, developing advanced reactors, and optimizing the culturing conditions for the high yield and productivity of BC, with the aim to minimize its production cost. Considering the applications, BC has attracted attention in highly diversified areas, such as medical, pharmaceutics, textile, cosmetics, food, environmental, and industrial sectors. This review is focused on overviewing the cost-effective synthesis routes for BC production, along with its noteworthy applications in the food and environmental sectors. We have made a comprehensive review of recent papers regarding the cost-effective production and applications of BC in the food and environmental sectors. This review provides the basic knowledge and understanding for cost-effective and scaleup of BC production by discussing the techno-economic analysis of BC production, BC market, and commercialization of BC products. It explores BC applications as food additives as its functionalization to minimize different environmental hazards, such as air contaminants and water pollutants.

Antibacterial aerogels with nano‑silver reduced in situ by carboxymethyl cellulose for fresh meat preservation

Bacterial cellulose (BC) was used as a reinforcing agent, citric acid (CA) as a cross-linking agent, and CMC@AgNPs as antibacterial nanomaterials, in which CMC@AgNPs were reduced from AgNO₃ in situ by carboxymethyl cellulose (CMC) as a reducing agent and stabilizer to fight microbial corruption. Its potential application in packaging fresh meat has been investigated. Results showed that the antibacterial CMC@AgNPs/BC/CA aerogels with excellent structural integrity and outstanding water absorption were developed by adding 0.3% BC and 0.25% CA. The CMC@AgNPs/BC/CA aerogel significantly reduced the color change and the total viable bacterial counts (TVC) in fresh meat after 7 days of refrigerated storage. The results indicated that CMC@AgNPs/BC/CA aerogels can effectively extend the shelf life of fresh meat, and can be used for meat packaging as a biologically active absorption pad.

Sustainable Cross-Linkers for the Synthesis of Cellulose-Based Aerogels: Research and Application

Cellulose aerogels with polyester resin as cross-linkers have attracted much attention. This study describes the route to produce a fully bio-based aerogel with high added value from waste paper and starch, cellulose acetate and starch–cellulose acetate mixture as cross-linkers for oil adsorption, instead of the environmentally harmful polyester resin. The manufacturing process is simple, sustainable and cost-efficient, without releasing harmful by-products into the environment. The effects of different cross-linkers on the oil adsorption, dynamic oil retention, reusability and morphology of the aerogels were studied in detail. Experimental results show that these environmentally friendly recycled aerogels have a very low density, i.e., —0.0110–0.0209 g cm−3, and highly porous structures, with a porosity of 96.74–99.18%. The synthesized hydrophobic aerogels showed contact angles of ∼124–129°. The compression moduli are lower than that of an aerogel with polyester as a cross-linker, but the compression modulus of the mixture of starch and cellulose acetate especially shows a higher value than expected. The sorption capacity of the aerogels with bio-based cross-linkers was significantly increased compared to the aerogels with polyester; it is now up to 56 times their own weight. The aerogels also have good oil-retention properties.

Hydrophobic composite foams based on nanocellulose-sepiolite for oil sorption applications

TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl)-oxidized cellulose nanofibers (CNF) were assembled to fibrous clay sepiolite (SEP) by means of a high shear homogenizer and an ultrasound treatment followed by lyophilization using three different methods: normal freezing, directional freezing, and a sequential combination of both methods. Methyltrimethoxysilane (MTMS) was grafted to the foam surface by the vapor deposition method to introduce hydrophobicity to the resulting materials. Both the SEP addition (for the normal and directional freezing methods) and the refreezing preparation procedure enhanced the compressive strength of the foams, showing compressive moduli in the range from 28 to 103 kPa for foams loaded with 20% w/w sepiolite. Mercury intrusion porosimetry shows that the average pore diameters were in the range of 30-45 µm depending on the freezing method. This large porosity leads to materials with very low apparent density, around 6 mg/cm³, and very high porosity >99.5%. In addition, water contact angle measurement and Fourier-transform infrared spectroscopy (FTIR) were applied to confirm the foam hydrophobicity, which is suitable for use as an oil sorbent. The sorption ability of these composite foams has been tested using olive and motor oils as models of organophilic liquid adsorbates, observing a maximum sorption capacity of 138 and 90 g/g, respectively.

Cellulose aerogel composites as oil sorbents and their regeneration

Background

With every oil tanker comes the risk of an accident and oil spill. Sorbents are the most suitable means to remove oil spills. Aerogels as sorbents have high porosity and can be made from cellulose from paper waste. The literature does not distinguish between paper and cardboard as sources of cellulose aerogels and little is known about composites of cellulose aerogels consisting of cellulose fibres and chemically untreated, unprocessed fibres or particles of straw, wool, macroalgae or cellulose acetate from cigarette butts. In this study, the sorption properties for marine diesel oil and biodiesel of such aerogels and their regenerative capacity with bioethanol were investigated.

Methods

Cellulose aerogels were prepared from office paper and cardboard waste without and with chemically untreated algae, straw, wool and cellulose acetate as a composite by freeze drying. All samples were hydrophobised with methylsilane. The density to calculate the porosity and the contact angle were determined. Then the sorption capacity was determined over five cycles of sorption of oil and regeneration with bioethanol.

Results

The average contact angle of all samples was 125°, indicating hydrophobicity. Paper-based aerogels were found to consistently have higher sorption capacities for biodiesel, marine diesel oil and bioethanol than cardboard-based aerogels. In particular, the wool/cellulose aerogel composite was found to have better sorption capacity for biodiesel, marine diesel oil and bioethanol than all other samples. The cellulose acetate/cellulose aerogel composite showed significantly higher sorption capacities than the paper and cardboard control samples (highest value is 32.25 g g⁻¹) only when first used as a sorbent for biodiesel, but with a rapid decrease in the following cycles.

Super-hydrophobic Cellulose Nanofiber Air Filter with Highly Efficient Filtration and Humidity Resistance

High-air humidity, especially condensation into droplets under the influence of temperature, can pose a serious threat to air purification filters. This report introduces the use of methyltrimethoxysilane (MTMS) for the silanization hydrophobic modification of cellulose nanofibers (CNFs) and obtains an air filter with super-hydrophobicity (CA = 152.4°) and high-efficiency filtration of particulate matter (PM) through the freeze-drying technology. The antihumidity performance of CNFs filters that undergo hydrophobic modification in high-humidity air is improved. Especially in the case of high-humidity air forming condensed water droplets, the increase in the rate of filtration resistance of the hydrophobically modified CNFs filter is much lower than that of the unmodified filter. In addition, the water-vapor-transmission rate of the hydrophobically modified filter is improved. More importantly, adding MTMS can regulate the porous structure of CNFs filters and improve the filtration performance. The specific surface area and the porosity of the filter are 26.54 m²/g and 99.21%, respectively, and the filtering effects of PM_1.0 and PM_2.5 reach 99.31 and 99.75%, respectively, while a low-filtration resistance (42 Pa) and a quality factor of up to 0.122 Pa^-1 are achieved. This work has improved the application potential of high-performance air-purification devices to remove particulate pollution and may provide useful insights to design next-generation air filters suitable for application in high-air humidity.

Superelastic Polyimide Nanofiber-Based Aerogels Modified with Silicone Nanofilaments for Ultrafast Oil/Water Separation

Nanofiber membranes via electrospinning with layered structures are frequently used for oil/water separation, thanks to their unique properties. However, challenges that involve nanofibrous membranes still remain, such as high energy consumption and unfavorable transport properties because of the densely compact structure. In this study, superelastic and robust nanofiber-based aerogels (NFAs) with a three-dimensional (3D) structure as well as tunable porosity were prepared using polyimide (PI) nanofibers via a freeze-drying process followed by the solvent-vapor treatment. The porous NFAs were further modified using trichloromethylsilane (TCMS) to generate silicone nanofilaments (SiNFs) on the surface of the PI nanofibers, which could enhance the hydrophobicity (water contact angle 151.7°) of the NFAs. The corresponding superhydrophobic NFAs exhibited ultralow density (<10.0 mg m^-3), high porosity (>99.0%), and rapid recovery under 80% compression strain. SiNFs-coated NFAs (SiNFs/NFAs) could also collect a wide range of oily solvents with high absorption capacities up to 159 times to their own weight. Moreover, surfactant-stabilized water-in-oil emulsions could also be efficiently separated (up to 100%) under the driving force of gravity, making it a promising energy-efficient technology. Additionally, SiNFs/NFAs maintained high separation efficiency throughout five separation-recovery cycles, indicating the potential of SiNFs/NFAs in the field of oil/water separation, sewage treatment, as well as oily fume purification.

Biorefinery Approach for Aerogels

According to the International Energy Agency, biorefinery is "the sustainable processing of biomass into a spectrum of marketable bio-based products (chemicals, materials) and bioenergy (fuels, power, heat)". In this review, we survey how the biorefinery approach can be applied to highly porous and nanostructured materials, namely aerogels. Historically, aerogels were first developed using inorganic matter. Subsequently, synthetic polymers were also employed. At the beginning of the 21st century, new aerogels were created based on biomass. Which sources of biomass can be used to make aerogels and how? This review answers these questions, paying special attention to bio-aerogels' environmental and biomedical applications. The article is a result of fruitful exchanges in the frame of the European project COST Action "CA 18125 AERoGELS: Advanced Engineering and Research of aeroGels for Environment and Life Sciences".