Termed “ageless bionanomaterial” by Dufresne,1 nanocellulose is a nanoscale material consisting either of cellulose nanocrystals (CNC, also called nanocrystalline cellulose) or cellulose nanofibrils (CNF, also called nanofibrillated cellulose) having exceptional chemical, mechanical, biological, optical and thermal properties.2 Potential applications range from transparent and foldable material in flexible energy and electronic devices,3 through carmaking using nanocellulose-reinforced polymer composites.4 Being biocompatible, chemically stable and hydrophilic, nanocellulose also has numerous potential biomedical new usages.5
Large-scale production of this versatile biomaterial so far has been limited by the demanding physical and chemical conditions, required first to separate the lignin from wood lignocellulosic biomass, and then to extract nanocellulose from the latter cellulosic fraction.1,2 Following the separation step using either acid-chlorite or alkaline treatment (and thus generating large amounts of wastewater), nanocellulose is generally extracted via acid hydrolysis (adding to the wastewater burden), steam explosion (with high energy consumption), enzymatically (requiring overly long extraction times), mechanically (high pressure homogenization, and ball milling methods) or by ultrasonication, with large energy demand.2
Comprised of cellulose I only, CNC has a low aspect ratio (length/diameter = 10-100), a tensile strength similar to that of aramid-fiber (10 GPa), and is produced via acidic hydrolysis of plant (wood, cotton, etc.) cellulose pulp. Its suspensions have liquid-crystalline properties. CNF has a high aspect ratio (length/diameter = 100-150), includes amorphous cellulose along with cellulose I, and is produced via mechanical processes. Its dispersions in water exhibit gel-like characteristics.
The industrial production of CNC from wood cellulose pulp using sulphuric acid has an estimated production cost ranging from $3632/t to $4420/t, with feedstock cost and capital investment being the major cost drivers.6 Yet, in the same year of these estimates (2017) for large scale production, CNC was reported to be sold at $1,000/kg.7
In 1998, Isogai and Sato successfully applied the polysaccharide selective oxidation process discovered by de Nooy to regenerated and mercerized cellulose8 in order to partly convert the primary alcohol groups of cellulose to carboxylates using catalytic amounts of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and sodium bromide with aqueous NaOCl as primary oxidant.9 Eight years later, the team in collaboration with Vignon discovered that native celluloses could be fibrillated in 3-5 nm nanofibrils by simple mechanical homogenization of the solution containing the TEMPO-oxidized cellulose.10 The electrostatic repulsions between the cellulose fibrils bearing the carboxylate groups cause the shear and dispersion of the nanofibrils under mechanical agitation.
Since 2017, the process is used by a large paper company in Japan to manufacture CNF in the form of nano-dispersed fibers with uniform fiber width of 3 to 4 nm starting from bleached wood pulp at two different paper mills.11 The company supplies the ‘Cellenpia’ product series using CNF as key ingredient to different industrial customers producing CNF-reinforced tires, paper barrier cups for beverages, personal care, hygiene, and cosmetic products. In 2019, the same company successfully developed a CNF-reinforced resin that was used to produce a demonstration car (the Nano Cellulose Vehicle)12 whose weight compared to a conventional car was reduced by around 10%.
Today, CNF is sold at a cost of $90-100/kg.13 This high cost is due both to the high cost of TEMPO as well as of processing the spent hypochlorite dilute solution containing the TEMPO catalyst. Separating nitroxyl radicals in solution is a multi-step, expensive process.14 Furthermore, TEMPO is a genotoxic ingredient15 whose concentration in any material suitable for biomedical use, must be lower than a low threshold of toxicological concern (i.e., 4 ppm).16
In order to lower production costs, cellulose feedstocks alternative to wood pulp have been widely explored. Available in over 100 million tonne yearly amount, waste orange peel - namely the main citrus processing waste (CPW) obtained from the orange juice processing industry - would be an ideal feedstock. Unfortunately, the routes to citrus nanocellulose starting from CPW based on enzymatic,17 microwave-assisted hydrothermal treatment,18 and acid hydrolysis,19 all present significant technical limitations.
For example, the nanocellulose fibrils obtained via multi-step microwave-assisted extraction of dried depectinated orange peel are deeply colored in brown due both to caramelized sugars and to the Maillard reaction between sugars and residual proteins at the high working temperatures required for extraction (120 °C to 180 °C).18
Now, we report that the insoluble fraction resulting from the hydrodynamic cavitation (HC) of citrus processing biowaste carried out on a semi-industrial scale in water consists of a new nanocellulose of high quality. Dubbed “CytroCell”,20 this citrus nanocellulose is readily obtained in large amounts through a one-pot process requiring no prior or subsequent chemical or mechanical treatment.