Sustainability 2023 , 15 , 6915
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abundant biopolymer, and it is used for paper, cardboard, and textile manufacturing [27]. It is a linear homopolymer of glucose and represents around 40–60% of wood mass. The second most abundant biopolymers on Earth after cellulose are lignin and hemicellulose, each representing around 20–30% of wood mass [28]. Lignin is a networked polymer of phenylpropane units, and it is responsible for the resistance and stiffness of wood in living trees [29]. It also gives wood its yellowish to brownish color because it is brown in color when extracted as powder, while cellulose and hemicellulose are white to off-white [28]. Hemicellulose is a branched heteropolymer of seven simple sugars: glucose, galactose, mannose, xylose, arabinose, glucuronic acid, and galacturonic acid [30]. It acts as the glue between cellulose and lignin in the structure of wood at the anatomic level [31]. The production of paper from wood involves several steps: wood pulping, pulp bleaching, purification, beating and refining, papermaking, and final treatments [3]. Being the first step in papermaking, the main goal of the pulping process is to remove lignin and hemicellulose while keeping the structure of the cellulose fibers intact to form pulp [32]. The efficiency of pulp production is dependent on the chemical composition of the wood and the manufacturing processes used for pulping. The variation in the lignin content of wood significantly affects pulping because lignin is the most recalcitrant wood component to remove [33]. Wood pulping methods can be chemical, mechanical, or semi-mechanical [34]. As the names indicate, chemical pulping uses chemicals to dissolve lignin and hemicellulose to produce the pulp fibers; mechanical pulping uses mechanical processes such as grinding and refining for that purpose [35]. The pulp produced by chemical methods is of a higher quality and contains a higher amount of cellulose fibers compared to the mechanically produced pulp. In other words, chemical pulping is more efficient in removing lignin and hemicellulose, leading to a pulp with higher cellulose content [36]. Chemical and mechanical pulps are advantageous for different applications. For example, mechanical pulp has excellent printing properties [37]. Along with papermaking, chemical pulp is used in other applications, such as the textile industry, pharmaceutical industry, and fuel production [38]. Kraft pulping and sulfite pulping are the most common chemical pulping methods, with Kraft pulping being the most commonly used [39]. In 2016, 95% of the paper mills in North America used Kraft pulping [40]. Environmentally speaking, numerous waste products, such as lignin and effluent sludge, are generated during Kraft pulping [41]. After wood pulping, bleaching takes place to further eliminate the remaining lignin [40]. It improves the whiteness of mechanical or chemical pulps by removing the chromophoric lignin groups using chlorine or chlorine dioxide [42]. The bleached pulp is then purified by removing any rejected cellulose fibers via filtration. Rejected fibers exhibit similar properties to bleached pulp but do not hold a strong place in the global market due to the presence of impurities. Still, they are sometimes used as raw materials for cardboard manufacturing [41]. The purified bleached pulp then goes through a beating and refining process to increase the surface area of its fibers, which in turn increases their ability to hold water and form better bonds before the papermaking process [35]. The papermaking process is mostly a pulp dewatering process, in which the pulp is passed through rollers or wire meshes, which assist in the removal of water and the formation of paper. Finally, the newly formed paper is post-treated by calendaring, supercalendering, sizing, laminating, impregnating, or saturating to smoothen the surface of the paper and increase its quality [3,43]. The PPI has a significant impact on the environment as the main pulp fibers required for paper come from trees. Millions of trees are cut down annually to suffice the material uptake of the industry [44], and it is known that deforestation leads to an increase in temperatures globally, a rise in sea level, and a loss of biodiversity [6]. Moreover, pulping and papermaking require massive amounts of water, and the discharged wastewater contains high levels of toxic chemicals that harm the environment [45]. For instance, bleaching requires the use of different oxidizing agents, such as chlorine dioxide and ozone, which are considered harmful to the environment. In order to mitigate their environmental harm, the pulp is sometimes bleached using other green agents such as hydrogen peroxide;
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