# Utilization of Natural Adsorbents in the Purification of Used Sunflower and Palm Cooking Oils
## 1 Introduction  
Waste cooking oil (WCO) is derived from different types of edible vegetable oils that are utilized for frying food in the food industry or by individuals [1]. The oxidation of lipids is affected by factors such as light intensity, temperature, and long-term exposure to oxygen [2]. Used cooking oils may contain harmful substances resulting during their heating, such as oxidative compounds, thermal degradation products, and chemicals from interaction between food components and oil. Initially, hydroperoxides and peroxides are generated, followed by the accumulation of peroxide polymers [3, 4]. An increase in peroxide value (PV) and free fatty acids (FFAs) has been reported after cooking oils are heated at different temperatures and time intervals. As the heating time increases, the quality parameters being investigated also increase, resulting in the degradation of the oil [5]. The consumption of degraded oils can have detrimental impacts on the health ofboth humans and animals [6], while inadequately disposing of degraded oils can lead to environmental pollution. [7]. The quality of the oil depends on its physical (color, viscosity, solidification temperature, consistency, and texture) and chemical (peroxide, iodine, acidity, and saponification values) properties [8]. Vegeto oil deterioration is most frequently attributed to rancidity, a process in which fats and oils in food break down, resulting in a disagreeable odor, modified flavor, and texture alterations [9]. This degradation occurs as unsaturated fatty acids oxidize when exposed to light, heat, or air [10]. The recommended limits established for quality oils are as follows: PV less than 3 when the oil leaves the factory, less than 5 after the bottle is opened, and less than 10 during use. However, in general, the values are substantially lower than these. Rancid odor only appears when the PV reaches values between 30 and 40 milliequivalents/kg [11]. For the classification of vegetable oils according to the content of TCP in the EU, there are recommendations that for fresh vegetable fats, they should be between 0 and 18%. The critical range is 19-24%, and over 24% is considered waste oil. Also, the Food Safety and Standards Authority of India recommends 15% TPC in use and a maximum limit of 25% for used ones [12]. Reducing the acidity of waste oils is an important requirement for their use in biodiesel production. To be used in biodiesel production, the AV must be between 1.5 and 2 mg KOH/g [13; 14]. One of the biggest health risks is environmental pollution. Besides the negative effects on the environment due to various sources of pollution [15; 16; 17], the improper disposal of WCO also results in economic losses [18]. However, in the contemporary sustainable economy and circular economy perspective, WCOs after prior purification can be recycled and can constitute a promising and sustainable raw material for various industrial products [19; 20]. The recycling of WCO into alternative cost-effective and highly reusable materials poses a significant challenge. However, the utilization of biosorbents offers an environmentally friendly solution that enhances economic sustainability [21]. Natural adsorbents, such as zeolites, are increasingly appreciated and used for their benefits and useful properties in the adsorption and filtration of chemical substances, ion-exchange capabilities [22], moisture absorption, plant growth, medical uses, uses in construction, and more [23]. Zeolites are natural minerals--aluminosilicates--that are found in volcanic rocks with extraordinary properties, after being subjected to a long transformation process, and with benefits mainly in the adsorption of harmful substances from air, water, and soil [24]. The aluminosilicates have a three-dimensional structure of SiO\({}_{4}\) and AlO\({}_{4}\) tetrahedra. On another note, eggshells contain CaCO\({}_{3}\), and the natural porosity contributes to the efficiency of the oil adsorption process [25].