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disposal [125]. Among these nanoparticles, cellulose nanocrystals (CNCs) are crystalline nano-rods with a thickness of 3–10 nm and a length of a few hundred nanometers. They are usually extracted from pulp fibers using an acid-mediated procedure, which has already been industrialized. They can also be extracted directly from wood and lignocelluloses using a variety of reagents and processes [28,126,127]. Cellulose nanofibrils (CNFs), an- other form of cellulose nanoparticles, are semi-crystalline spaghetti-like nanoparticles with a thickness of 5–30 nm and a length of a few micrometers. They are usually produced by the mechanical fibrillation of pulp fibers using a wide range of techniques, includ- ing microfluidization and homogenization [128,129]. The dimensions and crystallinity of nanocellulose are dependent on its origin and the used method of extraction [130,131]. In addition to their high mechanical properties, biodegradability, and high surface area, CNCs and CNFs are famous for the possibility of modifying their surfaces through abundant hy- droxyl groups [131–134]. Due to these interesting properties, CNCs and CNFs have shown great potential in a wide range of applications, including flexible electronics [135–137], energy harvesting materials [138], water treatment membranes [139], drug delivery sys- tems [140–142], tissue engineering [143], biodegradable packaging [144,145], oil clean-up materials [146], and lightweight composites for automotive industries [147]. Nanocellulose is usually extracted from wood pulp and plant fibers. However, several methods have been developed to produce it from paper waste and rejects. The most commonly used acid to obtain CNCs by hydrolysis is sulfuric acid (H 2 SO 4 ) due to its tendency to strongly isolate CNCs and disperse them as a stable colloid system by esterification of their surface hydroxyl groups. Reaction time, temperature, and acid concentration are the main factors that determine the properties of the CNCs [142]. Following acid hydrolysis, CNCs are washed with water to halt the reaction and are then recovered by centrifugation. Due to environmental concerns, acid hydrolysis cannot be implemented in a biorefinery scheme. To overcome this, other routes of obtaining nanocellulose, such as enzymatic hydrolysis, have surfaced. Cellulase is the most commonly used enzyme class for enzymatic hydrolysis due to its ability to attack cellulose fibers with high selectivity [129]. The main drawback of acid hydrolysis is harsh conditions, while enzymatic hydrolysis takes significantly more time. Therefore, subcritical water treatment has been explored because it does not require harsh conditions and an extended period of time [148]. The CNCs and CNFs extracted from paper waste can be used in the papermaking process. CNFs can be utilized as ad- ditives to promote adhesion between the fibers and to fill the voids in the paper, which could result in improved strength of the paper in its final form [149,150]. This is because nanocellulose has a large surface area due to its nano-size and many free hydroxyl groups, promoting hydrogen bonding and stronger structure formation [151]. The major issue with the extraction of CNCs and CNFs from paper waste is the calcium carbonate that exists in paper waste. It is expected that it interferes with the extraction process in terms of the formation of by-products and reduces nanocellulose yield and purity [24,152,153]. Still, CNCs and CNFs extracted from paper waste have been explored in a wide range of applications [131,152,154]. 4.5. Hydrochar by Carbonization Paper mill sludge has a high moisture content, which increases drying and trans- portation costs limiting its utilization. High moisture also attracts microorganisms, which digest the organic fraction of the sludge, resulting in the release of greenhouse gases into the environment [155]. The organic fraction of paper sludge is suitable for hydrothermal carbonization to produce carbon-based materials. Hydrothermal carbonization involves the conversion of wet biomass compounds to high carbon-containing solid products in sub-critical water conditions [156,157]. It usually takes place in a temperature range of 160 ◦ C to 180 ◦ C and self-generated pressure in the range of 1 to 3 MPa and results in a material similar to coal, known as hydrochar. Hydrochar has interesting properties, such as improved hydrophobicity and better dewatering properties compared to paper waste. When dried, its calorific value increases compared to the initial feedstock. Hydrochar can
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