Adsorption is widely acknowledged as one of the best options that are available for the removal of contaminants from water. Contamination of water does not only create water scarcity, but it has the capacity to generate and transfer several environmental problems including threat to public health. This chapter reviewed calcium oxide nanoparticle (CaONP) as a noble metal oxide for the removal of contaminants from water. The review is concentrated in the general overview of water contamination, metal oxide nanoparticles, general application of CaONP, synthetic methods, characterization method, and applications. The chapter observed that little is done on the use of CaONP for the removal of contaminants from water except for dyes, some heavy metal ions, and few organic/inorganic compounds. It is also observed that CaONP can be applied as adsorbent and in photocatalytic degradation of dye. Suggestions are made on the possibility of utilizing local raw materials that are easily accessible, cheap, and environmental sources of raw materials for the synthesis of CaONP.
TopIntroduction
According to a World Health Organization information sheet, by 2025, half of the world's population would be living in water-stressed regions. In the least developed nations, 22 percent of healthcare institutions do not have access to water, 21 percent do not have access to sanitation, and 22 percent do not have access to waste management (Boretti & Rosa, 2019). Contamination of different sources of water has taken different dimensions and the final consequence is in the form of polluted water. The effect of water contamination on the ecosystem has prompted considerable attention for contamination of water by various forms of physicochemical parameters, heavy metals, emergent contaminants, nutrients, etc. (Njuguna et al., 2020). The presence of unwanted contaminants in water can affects aquatic life and alter the quality of water against its normal usage. Hence, water remediation involving the removal of unwanted and harmful contaminants is a must before its use for domestic, agricultural, and industrial use (Borgohain et al., 2020). Depending upon the nature of contaminants and feasibility of the process, several options are available for mitigating water pollution, involving reverse osmosis, filtration, oxidation, adsorption and biological treatment, etc. (Dave & Chopda, 2014). However, the use of the adsorption process has gained wider acceptance because it is cost-effective and can be implemented with biodegradable materials to remove contaminants from wastewater (Sarwar et al., 2021).
Adsorption involves the sticking of particles (i.e., adsorbates such as molecules, ions, atoms, etc) on a surface (called adsorbent). The adsorption process can be influenced by temperature, pressure, nature of adsorbent, the concentration of adsorbate, pH of the solution, the mass of adsorbent, and other factors (Danesh et al., 2021). Consequently, processes that control adsorption must be based on the control of these factors. Different adsorbents have been studied for their ability to remove pollutants from water including plant samples (Tomar et al., 2014), eggshells (Lunge et al., 2012), agricultural wastes (Bansal et al., 2009) such as rice husk (Chuah et al., 2005; Vijila et al., 2020), sawdust (Bansal et al., 2012), charcoal (Mishra & Patel, 2009), cow dung (Rajkumar et al., 2019), diatomite (Akafu et al., 2019), brick powder (Yadav et al., 2006), polymers (Solanki et al., 2021), chitosan (Kamble et al., 2007), metal oxides (Srivastav et al., 2013), etc. However, most of the known adsorbents suffer from factors such as renewability, formation of stable product with the adsorbate, thermal instability, solubility, and other factors that work to reduce their efficiency (Xia et al., 2013). On the other hand, nano-sized adsorbents or nano-adsorbents are highly effective not only for the removal of heavy metal ions and degradation of organic compounds but also for disinfecting harmful micro-organisms (Oprčkal et al., 2017). Therefore, the applications of nano-adsorbents for adsorption/removal of contaminants in water have garnered the interest of researchers across the world.
Nanomaterials with sizes ranging in nano-scale (1-100 nm) are extensively utilized in diverse fields, including environmental monitoring, biomedical and pharmaceutical sector, electronics and optoelectronics, textiles, and cosmetics (R. Sharma et al., 2020). The nano-scale causes a variety of changes in the physical characteristics of the nanomaterials, including an increase in the volume to surface area ratio and the impact of the quantum effect on the particle size (M. Kumar et al., 2021). The characteristics of nanoparticles, such as their magnetic, optical, and electrical properties, are substantially different from those of conventional bulk materials. Nanomaterials have unique characteristics such as high adsorption, catalytic activity, and reactivity, as a result of their vast surface area and compact size (Garg et al., 2020). Several studies have shown that nanoparticles are effective in the remediation of wastewater. Nanomaterials have been used for water remediation including various pollutants such as emerging pollutants, organic pollutants, and inorganic anions in addition to disinfection of water (Jaiswal et al., 2021). Literature reports the use of various nanomaterials with potential applications in wastewater remediation. These include carbon nanotubes, zerovalent nanoparticles, metal oxide nanoparticles, and nanocomposites, among others. Literature also reveals that among the various metal oxide nanoparticles (such as TiO, MgO, ZnO, etc), calcium oxide nanoparticles (CaO NP) are the most demanded because of their biocompatibility (Vanthana Sree et al., 2020).