Microcrystalline cellulose modified by phytic acid and condensed tannins exhibits excellent flame retardant and cationic dye adsorption properties
Graphical Abstract
Introduction
Cellulose is a kind of renewable biopolymer from abundant plant sources as well as agriculture and forest wastes (Janaswamy et al., 2022). Traditionally, cellulose and its derivatives are widely used in papermaking, textile, food, beverage, medicine, pharmaceutical, chemical, environment, architecture and other domains (Rana et al., 2021d). Among of them, the textile and paper industries consume an enormous amount of cellulose every year. In recent years, cellulose products in novel physical forms such as microcrystals, nanocrystals, microfibrils, nanofibrils, nanowhiskers and aerogels have become one of the hotspots due to their unique properties, and found wide applications in diverse fields (Jiang et al., 2021, Omran et al., 2021, Rana et al., 2021a, Samir et al., 2005, Zhang et al., 2022). However, due to its lack of functionality, such cellulose must be functionalized or made into composites with other materials in order to upgrade its properties and broaden its applications (Eyley and Thielemans, 2014, Ghasemlou et al., 2021, Kalia et al., 2014, Tang et al., 2017). In this regard, the use of chemically modified cellulose products as flame retardant (FR) additives and adsorbents for the removal of dyes and heavy metal ions is two important application domains.
In the aspect of the FR functionalization of cellulose in novel physical forms, the phosphorylation is the most extensively used strategy (El-Shafei et al., 2019, Ghanadpour et al., 2015, Kokol et al., 2015). Classical phosphorylating agents are phosphoric acid and phosphate salts. The phosphorylation proceeds more favorably in the case of the use of molten urea to swell cellulose and catalyze reaction (Ghanadpour et al., 2015, Kokol et al., 2015), but such phosphorylation has some shortcomings such as the harsh reaction condition (135–160 °C) and the strength loss of cellulose (Bezerra et al., 2016, Yurkshtovich et al., 2007). Phosphorylated cellulose can become an intrinsically FR material (Ghanadpour et al., 2015). In addition to phosphorylation, silylation (Kim et al., 2018), borylation (Tong et al., 2021), and layer-by-layer coating by chitosan and phosphorous compounds (Köklükaya et al., 2017, Lou et al., 2021), can also impart FR function to cellulose. At present, FR microcrystalline cellulose (MCC) has been used as FR and reinforcing agents in epoxy and polylactide resins (Costes et al., 2016, Wang et al., 2020). FR nanocellulose and cellulose nanofibrils have found wider applications in textile finishing (El-Shafei et al., 2019), papermaking (Zhang et al., 2020), polyurethane foam coating (Carosio et al., 2018, Li et al., 2013), polymer electrolytes for lithium ion batteries (Gou et al., 2021), foam building materials (Ghanadpour et al., 2018), and lightweight, thermally insulating and FR aerogels for multiple uses (Yang et al., 2017).
As cellulose is available from abundant renewable biomass at low cost, its chemically modified derivatives and composites products are widely used as adsorbents for the removal of organic pollutants (e.g., dyes) and heavy metal ions (Varghese et al., 2019). Cellulose products in novel physical forms for adsorption applications have been reported (Fan et al., 2019, Sun et al., 2016, Zheng et al., 2018). In comparison to pristine cellulose, cellulose in micro- and nano-meter scale has a higher surface area, which is beneficial to adsorb pollutants. By changing the chemical groups and charge properties of cellulose and its composites, the adsorbents with different adsorption characteristics and applications can be obtained (Rana et al., 2021b, Rana et al., 2021c). Cationization is usually employed to render selective adsorption behavior of anionic dyes to cellulose (Feng et al., 2020, Zheng et al., 2018), while the introduction of anionic groups into cellulose offers selective adsorption ability of cationic dyes (Fan et al., 2019, Sun et al., 2016). Most notably, phosphorylated MCC exhibits excellent adsorption capacity towards both cationic dyes and metal ions (Hadid et al., 2021, Liu et al., 2015).
Another strategy to enhance the adsorption capability of cellulose towards dyes and metal ions is to blend it with other natural compounds such as chitosan, tannins and phytic acid (PA). The combination of cellulose and chitosan improves adsorption capability towards anionic dyes (Zheng et al., 2018). Tannins are easily available biosorbents that have high affinity to dyes and heavy metal ions (Bacelo et al., 2016). The immobilization of tannins onto microfibrillated cellulose exerts positive impact on the removal of cationic dyes (Wang et al., 2019). In particular, the combination of anionic cellulose and tannins exhibits great enhancement in adsorbing cationic dyes (Zou and Peng, 2018). PA treated wheat straw has good adsorption capability towards cationic dyes (You et al., 2016).
PA is rich in phosphorus with six phosphate groups. PA can catalyze the pyrolysis and carbonization of polymeric materials (Daneluti and Matos, 2013, Sykam et al., 2021), and consequently has great potential in the use of a FR agent. PA has been used in the FR treatment of cellulosic materials (Sykam et al., 2021). Our previous study revealed the feasibility of the phosphorylation of MCC using PA at 90–95 °C (Yuan et al., 2021), which is a mild reaction condition compared with the harsh condition of high temperature (135–160 °C) for the classical phosphorylation using phosphoric acid and phosphate salts (Bezerra et al., 2016, Yurkshtovich et al., 2007). Furthermore, PA is a kind of renewable biomass material compared with phosphoric acid and phosphate salts from phosphate rock. Therefore, the application of PA in the synthesis of FR agents can meet the requirements of green chemistry. On the other hand, condensed tannins (CT) have good thermal stability thanks to the presence of aromatic moiety, and are used in heat insulation and FR fields including textiles (Basak et al., 2021, Duval et al., 2017, Tondi et al., 2009).
Inspired by the phosphorylation of cellulose, the potential application of PA and CT in FR materials and the effect of PA and CT on the adsorption properties of cellulose, we tried to combine the modification of PA and CT on MCC, and hoped to obtain the novel MCC-based material with enhanced FR and adsorption capabilities which could be used in FR and adsorption fields, respectively. To this purpose, MCC was modified with PA and CT at a reflux temperature, and a product PA/CT/MCC was obtained. The FT-IR and phosphorus content analyses were used to confirm the phosphorylation of MCC. The thermal stability, charring ability, heat release capability, and combustibility of PA/CT/MCC were studied. Moreover, the adsorption capability of PA/CT/MCC towards a commercial cationic dye from water and the adsorption mechanism were discussed.
Section snippets
Materials
Microcrystalline cellulose (MCC, 50 µm) was bought by Shanghai Meryer Chemical Technology Co. Ltd., China. Condensed tannins extracted from grape seeds (CT, 95%) and phytic acid (PA, 70%) were bought from Xi’an Huike Biological Co. Ltd., China and Chengdu Ai Keda Chemical Technology Co. Ltd., China, respectively. Cyclohexane, hydrochloric acid, sodium hydroxide and urea were bought by Jiangsu Qiang Sheng Chemical Co. Ltd., China. Dicyandiamide was bought from Shanghai Ling Feng Chemical Reagent
Morphology, structure and surface charge characterizations
Fig. 1A depicts the SEM micrographs of MCC and PA/CT/MCC. MCC presented a typical feature of rod-like microstructure. Interestingly, PA/CT/MCC had a dramatic change in morphology, and it displayed an irregular block structure with some voids. This phenomenon is likely to be a consequence of the hydrolysis of MCC by PA (Niu et al., 2020). Such structure results in a larger specific surface area, which can lead to an improvement in adsorption capability.
Fig. 1B shows the FT-IR spectra of MCC, CT
Conclusions
In this study, PA and CT were introduced into the macromolecular chains of MCC to prepare an MCC-based novel material, PA/CT/MCC. Due to the interactions of PA and CT with MCC, the modified MCC material underwent great changes in chemical groups, morphological structure, charge nature, thermal stability, flammability, heat release property during the combustion, microstructure of burned char residue, and capability of adsorbing cationic dye.
The resulting PA/CT/MCC was negatively charged with a
CRediT authorship contribution statement
Hua-Bin Yuan: Methodology, Investigation, Data curation, Formal analysis, Writing – original draft. Ren-Cheng Tang: Conceptualization, Formal analysis, Supervision, Writing – review & editing, Supervision. Cheng-Bing Yu: Conceptualization, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
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