Current waste management strategies involve minimizing waste, as well as collection and storage/treatment [1, 2]. However, waste such as biomass generated from agricultural and domestic activities is inevitable [3]. These biomasses like eggshells are generated daily in enormous quantities since their main products are life-dependent: source of food [4, 5]. Although agro-biomass is biodegradable in the natural environment, the biomass contains vital compounds or elements that have many uses [6]. For instance, carbon found in biomass can serve as a precursor for producing fuels, gas, adsorbents, and several others [5, 7,8,9,10]. Hence, waste biomasses are valuable materials, and the valorization of such waste is sustainable, economical, and generally eco-friendly [11, 12]. Moreover, recycling waste offers a more cost-effective approach to managing waste and preventing environmental pollution [13].
Eggshells are industrial and household byproducts; thus, they are abundant and available at a low cost. The global egg production is around 77 million tonnes, resulting in over a million tonnes of eggshell biomass created as waste each year [14, 15]. However, the majority of eggshells are disposed in landfills [5]. Meanwhile, eggshell is a biodegradable material with extraordinary properties such as a unique natural porous structure and a high calcium carbonate (bio-CaCO3) concentration (95 wt%) in the form of calcite [16, 17]. Thus, eggshells can be used in various applications, including as a biosorbent for environmental treatment, where it has a strong affinity for heavy metal ions and dyes [18, 19]. The calcite in eggshell can also be used: in the production of biocompatible ceramic materials, as an abrasive ingredient in toothpaste, coating pigments for ink-jet printing paper, as bio-filler to improve the properties of polymer nanocomposites, to improve the mechanical behavior and tensile properties of polypropylene composites, controlled epoxy resin composite, and improve the thermal stability and glass transition temperature of normal corn starch foams [12, 17, 20, 21].
On the other hand, nanotechnology involves the conversion of bulk materials to nanometric size (< 100 nm) [22, 23]. Nanotechnology introduces unique characteristics into materials and makes them widely applicable as catalysts, structural components, information storage, electronics, and sensors [24,25,26,27,28]. Hence, nanomaterials have been the focus of current research [28, 29]. Studies have shown that the conversion of eggshells into nanometric size presents significant advantages over bulk or micrometer-sized eggshells [17, 30]. Nanometric eggshell has a relatively high surface area and uniform pore distribution. Therefore, nano-eggshell is used as a more efficient adsorbent for dyes and metals in solution and additive for plastics and ceramic composites to improve their properties [17]. Other studies have shown that nanometric eggshells can be more effectively deposited on the biochar matrix to improve the adsorption capabilities [18].
Meanwhile, several top-bottom methods can be used to synthesize nanomaterials [31]. High-energy ball milling is the most widely used method in producing nanomaterials because ball milling is simple, applicable for numerous materials, and can be advanced quickly for commercial production [32]. However, the comminution of particles from micrometer to nanometer scale by milling requires much energy and is costly [32]. Further drawbacks of milling include particle agglomeration and nanomaterial contamination [32]. Consequently, other methods such as ultrasonic irradiation have been employed as an effective alternative for synthesizing nanomaterials with remarkable properties [17].
Sonochemistry is an acoustic cavitation process that involves the creation, growth, and implosive collapse of bubbles in a liquid medium [33]. This result in extreme conditions such as high temperatures (> 5000 K), high pressure (> 20 MPa), and high cooling rate (> 107 K s−1) [34]. These extreme conditions introduce many unique properties in the irradiated solution, affecting the size reduction [34]. Sonication is a suitable method for reducing the particle size of many inorganic materials while preserving the crystalline structure [35]. A study by Hassan et al. (2013) investigated the preparation of bio-CaCO3 nanoparticles from eggshell using wet ball milling with polypropylene glycol. The ball-milled eggshell particles were then irradiated a sonochemical process in the presence of N,N-dimethylformamide (DMF), decahydronaphthalene (Decalin), and tetrahydrofuran (THF). DMF was reported as the most effective solvent [17]. Low volatile solvents like DMF have relatively low vapor pressures and are effective solvents for synthesizing biobased nanomaterials via the sonochemical process [17, 35].
Nevertheless, a crucial aspect of the nano-CaCO3 preparation via the sonochemical process which includes optimizing the mixing ratios between the eggshell powder and solvent has not been discussed [36,37,38,39]. Such study is vital to the large-scale production of the bio-CaCO3 nanoparticles from this process. This can specify the maximum eggshell/DMF ratio required to produce high yield eggshell nanoparticles via the sonochemical process. Moreover, studies have shown that the efficiency of sonochemical processes is critically dependent on the solid/solvent ratio [40].
This present study investigates the effect of eggshell/DMF mixing ratios on the sonochemical production of CaCO3 nanoparticles for large-scale applications. This is the first study that examines the eggshell/DMF mixing ratios and optimizes the ratios to maximize CaCO3 nanoparticle production. This work further discusses the effect of the mixing ratios on the size, crystallinity, and porosity of the CaCO3 nanoparticles.