The necessity for goods such as food and clothing drive the global demand for industrial agriculture production. As the growth of industrial agricultural production rises, the amount of waste production also increases. Areas of waste that are of devastating concern include plastics[1] and biowaste[2]. Looking to the future, it is imperative that sustainable methods to support human life are met. Currently, most biowaste is disposed of by dumping in a landfill or burning. These irresponsible practices impact greenhouse gasses and prevent nutrients from returning to the soil where they are most needed[3]. To increase renewability and create a circular economy[2], it is important to find responsible ways for disposing of industrial biowaste. One method of interest for the responsible disposal of biowaste is extraction of nanocellulose, resulting a value-added biproduct. Cellulose is the most abundant and renewable organic material produced on the planet with current estimates of cellulose production around \(1.5*{10}^{12}\) tons per year[4]. Cellulose is found in plants, bacteria, algae, and some animals (tunicates)[5], making it a virtually inexhaustible resource. In concert with lignin and hemicellulose, cellulose plays a critical role in maintaining the structural and mechanical characteristics of plants and tunicates[6]. Cellulose is a semicrystalline material composed of crystalline and amorphous regions. Crystalline regions can be separated from each other by breaking the amorphous regions, resulting in significantly small components known as Cellulose Nanocrystals (CNCs) which are considered as the building blocks of cellulose[5]. CNCs are needle like in structure with diameters averaging between 5–50 nm and lengths ranging anywhere from 100 to 500 nm[5] on average. CNCs have unique properties which make them valuable, which include large aspect ratios, low density, water insolubility[6], desirable mechanical properties[5], biocompatibility[5], biodegradable[5], renewable, non-toxic[7], and have a highly modifiable surface[5]. CNCs natural occurrence in plant cell walls makes them a great source for sustainable and environmentally-friendly material[8]. Therefore, there are many applications for CNCs including: antimicrobial/antiviral systems, tissue engineering, drug/gene delivery, modified drug release, biosensors, protein scaffold/biocatalyst, enzyme immobilization, water purification/waste-water treatment, supercapacitors, conductive films, sensors, energy storage, biodegradable plastics, food storage, and much more[9]–[11].
Current sources used for CNC production include, but are not limited to, cotton, alfalfa tunicate, wheat straw, rice straw, sugarcane bagasse, and algae. Recently, the ‘2018 US Farm Bill’ has enabled the legal growth of hemp that has some of the highest cellulose content (70–78%) next to cotton (65%)[12]. Hemp crop can be grown in more states in the U.S. than cotton because it requires less fertilizers, herbicides, insecticides and water. Further, hemp provides higher annual yields than cotton even with the requirement of little to no agrochemical input[13]. A rapid and massive increase in hemp production has been seen in the United State, resulting in both opportunities and challenges. One of challenges is associated with agro-wastes from hemp production. Current estimates for annual biowaste production from hemp are around 150 million tons and, as of now, there are four methods used for disposal: dumping in a permitted landfill, composting, in-vessel digestion, and incineration[14]. This large amount of biowaste production and lack of responsible disposal highlights hemp agro-waste as a drastically underutilized potential source for CNC synthesis. Even though CNCs are not a new nanomaterial and numerous research has been done on these nanoparticles, there has been very little research into hemp sourced CNCs[15].
CNCs can be extracted using various methods: subcritical water extraction[16], ball milling[17], high-intensity ultrasonication[18], acid hydrolysis, Ionic liquid[19], oxidation via TEMPO with homogenization[20], and oxidation using Ammonium persulfate[21]. The method of extraction and the source create variability in CNCs that can impact their potential utilization. The most common method for CNC extraction reported is acid hydrolysis, however, this method is not optimal. Acids create lots of hazardous waste, they are corrosive, and hard to scale up for production of CNCs. Mechanical methods require large amounts of energy making them cost inefficient and hard to scale up for mass production. More environmentally friendly methods have been studied, such as subcritical water extraction and enzymatic extraction. In 2011 the first article using Ammonium Persulfate Oxidation (APS) was published, highlighting the use of a one-step method for CNC synthesis[21]. Since its original publication, the use of APS oxidation has increased in popularity due to it being cheaper, more eco-friendly, having low toxicity, high solubility, no need for pretreatment, using a simple production process, and it being less corrosive than other methods[22]. Upon heating APS, the peroxide bond is cleaved resulting in two \({{SO}_{4}}^{-}\) radical ions; simultaneously hydrogen peroxide (\({H}_{2}{O}_{2}\)) is formed. Together these compounds oxidized \({CH}_{2}OH\) located at the C6 position, breaking apart the amorphous regions of cellulose[23] and releasing the crystalline regions of cellulose. This one-step method not only requires fewer chemicals, the byproducts from the reaction process can be recovered allowing for responsible waste management[23]. Being able to recover waste products reduces hazardous waste and introduces the potential for large scale production making it a more environmentally friendly procedure that can support the extraction of CNCs from biowaste.
The work reported hereby successfully demonstrates the extraction of CNCs from hemp agro-waste using a one-step Ammonium Persulfate (APS) method and further obtains understanding of mechanical properties of these CNCs derived from hemp agro-waste. There have been published studies conducted on the extraction of CNC from hemp using chemical methods including APS, as well as studies on the mechanical properties of CNCs from different sources using various methods. However, to date there have been no focused studies conducted on CNCs sourced from hemp, including those sourced via the one step APS method. Furthermore, there have been no studies on the mechanical properties of hemp sourced CNCs, especially using nanoscale characterizations of individual CNC. The source and method of CNC synthesis impacts the mechanical properties of this nanomaterial, which in turn can affect its real-world applications. Analysis of CNCs show that APS treatment of hemp agro-waste preserved the morphological characteristics of these nanoparticles. Specifically, mechanical analysis using Force Distance Spectroscopy was used to determine Young’s modulus, Adhesion Energy, and Maximum load, which are in agreement with prior findings showing that APS treatment does not change the CNC structure. These results infer that hemp agro-waste is a viable option for applications in many areas, such as biomedical devices. In conjunction with the ability to recover waste byproduct materials, discussed in previous literature[23] using a post processing method, this study suggests the potential of decreasing the ecological impact of industrial hemp growth/waste in America by using it as a source of CNC synthesis.