Lignin based nano-composites: Synthesis and applications
Introduction
In the recent years, biodegradable resources have gained much interest in the enrichment and maintenance of industries (Dionisi et al., 2005). Considering the environmental complications like unavailability and other problems related to the usage of petroleum like non- renewable sources, we are forced to find an alternative in chemical industries (Dale, 2003). Lignin, a wood component is considered a viable option to sort out these problems. In view of the current environmental and energy issues, lignin has gained added attention as a renewable biomass resource. Nowadays, it is exclusively used for energy generation and is considered as an excellent alternative to many traditional fillers for various other applications (Kai et al., 2016; Liu, 2010; Kuhad and Singh, 1993).
In terms of chemistry, lignin can be considered as an organic complex biopolymer containing numerous polyphenols. It is the second most polymeric material of biological origin in abundance in nature and finds potential application in green technology. Thulluri et al. (2016) proposed the term “lignocellulose”, as it originates from lignin and cellulose which are the two major constituents in the cell wall of plants. Ligno-cellulosic substance contains 10−30 wt% of lignin and 40 % by energy. Hemicellulose is also seen in plant cell wall along with lignin and cellulose (Gao and Fatehi, 2019). These three components form an organic network by using intermolecular bridges, covalent bond and van der Waals forces. Lignin is acting as a binding component in plant cell walls by filling the space between hemicellulose and cellulose. Lignin can be easily separated out from agricultural and wood residues and it is also possible to separate lignin by precipitation from black liquor, which is a waste product obtained after the digestion of paper pulp. Being a bio-adsorbent it shows better adsorption efficiency, the soda-lignin obtained after the cooking process of pulp is more close to natural lignin and it can be precipitated easily by adjusting the pH to 13 by a two way carbonation process using impure CO2 obtained from flue gas as well as pure CO2 (Alén, 2015; Yue et al., 2016). Lignin contains various polar functional groups, which can take part in chemical bonding. Because of these characteristics, lignin can be effectively used for the adsorption of various pollutants and heavy metals (Ali et al., 2012; Babel and Kurniawan, 2003; Peternele et al., 1999; Wu et al., 2008). For instance, high adsorption capacity of lignin makes it an efficient candidate for the removal of chromium ions. Due to the ion-exchange mechanism, Cr(III) gets adsorbed on lignin and its concentration increases with increasing pH. An inner-sphere complex will be formed between Cr(III) and lignin and this adsorption process follows pseudo-second-order kinetics (Wu et al., 2008). Thus environmental friendliness, low cost and high availability makes this wood waste material a suitable adsorbent for waste water treatment. Moreover, lignin has enhanced reinforcing ability which enables its utility as a filler in the fabrication of composites.
In the current research scenario of nanotechnology, lignin-related nanomaterials have attracted wide industrial and academic attention (Chatterjee and Saito, 2015). The manipulation of lignin into micro and nanostructures could offer advanced performance and it can be synthesized using feasible conventional equipment (Sameni et al., 2020). It also possible to synthesize lignin nanoparticles with tunable properties for enhancing the functional applications (Richter et al., 2016). Due to the presence of the various chemical moieties as well as distinct network structure, lignin exhibits functional properties, such as UV-absorption (Gutiérrez-Hernández et al., 2016; Yearla and Padmasree, 2016), biodegradability (Tuomela et al., 2000), stabilizing effect (De Paoli and Furlan, 1985), reinforcing effect, anti-fungal and antibiotic activity (Pouteau et al., 2003; Barclay et al., 1997). Hence the conversion of lignin into nano dimension and translating it into nanocomposites could certainly open new possibilities towards the creation of novel materials with exciting properties. Nevertheless, the development of lignin nanocomposites is a challenging game due to the complex structure and high molecular weight of lignin molecule. However, many works were carried out recently for developing polymer nanocomposites in which lignin plays the role of a reinforcing agent (Pouteau et al., 2003; Hirose et al., 1998). A thorough analysis of literature reveals that lignin has been used as filler in the preparation of various types of composites, whereas the nano form of lignin has not been used predominantly. Hence the present review tries to summarize the synthesis, characterization and applications of various lignin based nanocomposites in an attempt to unveil the immense potentials hidden in this bioresource.
Section snippets
Synthesis of lignin nanoparticles
Nanoparticles (NPs) which are synthesized from renewable sources are biodegradable and have potential applications in various fields especially in agriculture. Cellulose, lignin and other biopolymers are under the category of renewable natural resources which can be utilized to synthesize nanoparticles for reinforcing composites (Gupta et al., 2014; Eichhorn et al., 2010; Zhang et al., 2008). Lignin NPs (LNPs) can be prepared by numerous techniques such as, flash precipitation, CO2 saturation
Functionalization of lignin nanoparticles
Reactivity of lignin is influenced by its origin, structure, chemical modification and the functionalities present in it. The meagre solubility of bare lignin in aqueous medium limits its large scale industrial production. Owing to the complexity in the molecular structure of lignin and the steric hindrance associated with its structure, the reactivity of lignin is restricted. But the plethora of functional groups in the structure makes it viable for surface functionalization. The modification
Lignin nanocomposites
Nanocomposites are basically a type of composites that have a 1–100 nm ultrafine phase dimension. Due to the nanosize effect, these materials exhibit excellent properties when compared to other conventional composite materials. Most of the reported works of polymer nanocomposites have synthetic non-biodegradable origin. There are only limited works based on bionanocomposites like chitin or cellulose based nanocomposites (Gopalan Nair and Dufresne, 2003).
Development of lignin based research opens
Characterization of lignin nano composites
Nano-lignin based biocomposites are characterized using different techniques to understand theinherent changes that occurin their structure during synthesis. The morphological analysis of the nanoparticles can be carried out by techniques such as, Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM) etc. X-ray Diffraction (XRD) gives information about the crystallographic information and its structure.Fourier Transform Infrared Spectroscopy
Applications of lignin nanocomposites
In the present situation of increasing environmental pollution, lignin as a biopolymer is attaining greater demand in various fields like pharmaceuticals, blends, composites, binder, dispersant, batteries, due to its economical, mechanical and adhesive properties. Around 98 % of lignin is consumed through combustion as fuel material; still the waste derived from the resources creates huge environmental impacts. So, the derivation of high end products from industrial lignin is a highly relevant
Conclusion
As the need of biocompatible materials are of prime interest in the present research scenario, the development and applications of lignin biopolymer based nanocomposites are considered vital.The review mainly portrays the synthesis methods, functionalization, characterization as well as the application of nano-lignin based composites. Various methods for the preparation of LNPs are discussed in a comprehensive manner. Since the current era is facing a scarcity in petroleum and petroleum based
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgment
Authors would like to thank Amrita Vishwa Vidyapeetham, Amritapuri for the financial Support.
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