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Article

Utilization of Natural Adsorbents in the Purification of Used Sunflower and Palm Cooking Oils

by
Roxana Bostan
1,
Mirel Glevitzky
1,2,*,
Simona Varvara
1,
Gabriela-Alina Dumitrel
3,
Gerlinde Iuliana Rusu
3,
Maria Popa
1,
Ioana Glevitzky
2 and
Mihaela Laura Vică
4
1
Faculty of Informatics and Engineering, “1 Decembrie 1918” University of Alba Iulia, 15-17 Unirii Street, 510009 Alba Iulia, Romania
2
Sanitary Veterinary and Food Safety Directorate of Alba County, 7A Lalelor Street, 510217 Alba Iulia, Romania
3
Faculty of Industrial Chemistry and Environmental Engineering, Politehnica University Timisoara, 2 Victoriei Square, 300006 Timisoara, Romania
4
Department of Cellular and Molecular Biology, “Iuliu Hațieganu” University of Medicine and Pharmacy, 6 Louis Pasteur Street, 400012 Cluj-Napoca, Romania
*
Author to whom correspondence should be addressed.
Appl. Sci. 2024, 14(11), 4417; https://doi.org/10.3390/app14114417
Submission received: 22 April 2024 / Revised: 15 May 2024 / Accepted: 21 May 2024 / Published: 23 May 2024
Browse Figures
Versions Notes

Abstract

:
The purification of used oils and their introduction into production cycles lead to reduction in environmental contamination. A simulation was conducted to study the thermal degradation of sunflower oil under varying temperatures over time. In the purification process of used cooking oil, an adsorption technique using zeolite and eggshell as an adsorbent (5, 10, 20, and 30 g/100 mL used oil) was applied. To optimize purification, different doses of thermally and chemically activated adsorbents were used, at different temperatures (30 and 80 °C). Therefore, this study was conducted in batch operations to determine the effect of suitable adsorption for a contact time of the adsorbent of 2 h. In comparison, the purification of used vegetable oils was achieved using a saline solution. The adsorption capacity was evaluated by determining the physicochemical parameters of the oils before and after purification. The characterization of natural adsorbents was carried out using scanning electron microscopy (SEM) and X-ray fluorescence spectrometry (XRF). The results showed that the adsorbent in a dose of 30 g of zeolite activated with NaOH and heat-treated eggshell, respectively, exhibited a larger surface area and greater adsorption capacity. Adsorption increased with contact time. The FT-IR spectra of the oils showed the IR bands at 1097, 1160, and 1237, corresponding to the presence of the ester C-O-, 1743 for C=O ester, 2853, 2922 for (CH2, CH3), and 3008 for (C=C). The acidity and peroxide values decreased with increasing dose and contact time with zeolite or eggshell. Together, our result strongly suggests that natural adsorbents contribute to the purification of used 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 of both 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]. Vegetable 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 SiO4 and AlO4 tetrahedra. On another note, eggshells contain CaCO3, and the natural porosity contributes to the efficiency of the oil adsorption process [25]. Eggshell has biosorbent potential which can be used to remove both dissolved and dispersed oil. Different studies present it as a good adsorbent in the filtration process [26,27].
This article is a contribution to the field of research on oil degradation, exploring the potential of adsorbents like zeolite or eggshell for oil purification to enable reuse. This study compares these adsorbents in different doses with a chemical method of purifying oils with sodium chloride. SEM/EDX and XRF analyses were performed for the surface morphology and to characterize the composition of the adsorbents. Through FTIR spectral analysis, distinct variations in the spectral regions in the characteristic bands for both fresh and used oils were observed. The results obtained are considered adequate for establishing causal relationships and forecasting the trends in oil degradation and purification.

2. Materials and Methods

2.1. Samples of Vegetable Oils

In this study, the materials used were sunflower oil and palm oil obtained from the local market in Alba Iulia, Alba County, Romania. All samples were transported under the same conditions, stored in the dark at ambient temperature until testing. Table 1 summarizes the treatments that were applied to the examined fresh or used oil samples.
To determine the variation in the PV of sunflower oil depending on the temperature, the degradation of the oil was monitored in time through the heat treatment of a mix of animal (lean chicken breast) and non-animal (the size of the sliced potatoes was approximately 1.5 cm × 1.5 cm × 8 cm) foods at 5 different temperatures between 65 °C and 180 °C. The product of animal origin used was refrigerated vacuumed chicken breast from Avicola Buzău, Romania, purchased from the local market.
During the frying of the food mix, the temperature was kept constant at 65, 90, 120, 150, and 180 °C, respectively. Foods were replaced every 10 min, and samples were taken at the same intervals. The oil samples were brought to room temperature before analysis.
Before analyzing the WCO, they were filtered through a 400-micron mesh filter. The solid samples of palm oil were heated, with stirring, on a water bath at 45 °C.

2.2. Physicochemical Analysis of Vegetable Oil Samples

Moisture content. About 5–20 g of the sample was accurately weighed to 0.0002 g in a flask brought forward to constant mass. It was then placed in an oven at 101 ± 1 °C for 15 min, followed by cooling in a desiccator. The drying process was repeated until a constant weight was achieved [28].
Relative Density. The density was measured at 25 °C using a 50 mL bottle pycnometer, which had been washed with degreasing solution, water, and petroleum ether [29].
Refractive Index (RI). It was measured in triplicate with a Cell RX4 (Mettler Toledo, Shah Alam, Selangor, Malaysia) device [30].
Peroxide value (PV). The oil sample (2.5 ± 0.1 g) was mixed with 10 mL of chloroform until complete dissolution; then, 15 mL of acetic acid and 1 mL of saturated potassium iodide solution were added. The sample was stirred for one minute and left to stand for 5 min. Then, 75 mL of distilled water was added, and the released iodine was titrated in the presence of starch, under continuous stirring, with 0.01 M sodium thiosulfate solution. All measurements were carried out in triplicate [31].
pH determination. The pH was measured using the pH-meter PHT 810 (1340–5810) (EBRO, Freiburg im Breisgau, Germany). All measurements were performed in triplicate.
Viscosity. Viscosity was determined utilizing the viscometer microVISC—M (RheoSense Inc., San Ramon, CA, USA) at 40 °C. All measurements were conducted three times.
Acid Value (AV). A 5 ± 0.01 g vegetable oil sample was dissolved in 50 mL of solvent while being stirred. (ether: 95% alcohol (Chempur, Piekary Śląskie, Poland)). The resulting mix was titrated with 0.1 M KOH solution (Merck, Darmstadt, Germany) in the presence of phenolphthalein (Merck, Darmstadt, Germany) until the pink turning point. The result was expressed in mg KOH/g [32].
Iodine value (IV)—Hanus method. An amount of 0.2 g of oil was dissolved in 20 mL of chloroform (Merck, Darmstadt, Germany). An amount of 25 mL of Hanus solution was added (10 g of iodine monobromide (Merck, Darmstadt, Germany) in 500 mL of glacial acetic acid (Chempur, Piekary Śląskie, Poland). After stirring, it was kept in the dark for 30 min. Ten mL of potassium iodine was added to the 15% flask solution (Merck, Darmstadt, Germany) followed by 100 mL distilled water. The mix was titrated with 0.1 M sodium thiosulfate solution (Univar, Seven Hills, Australia) until the yellow straw color was achieved. Starch was added (Merck, Darmstadt, Germany), and the titration continued, until the blue color disappeared. Two blank samples were performed in parallel with the same amounts of solvent and Hanus reagent but without oil. The result was expressed as grams of I2 absorbed by 100 g oil [33].
Saponification value (SV). Two g of dry sample were mixed with 25 mL of the alcoholic potassium hydroxide solution (Merck, Darmstadt, Germany). An air cooler was installed, and the solution was boiled under reflux until complete saponification (about 1 h). The sample was titrated with 0.5 M hydrochloric acid (Chempur, Piekary Śląskie, Poland), in the presence of phenolphthalein (Merck, Darmstadt, Germany), until the red color disappears [34].
Total polar compounds (TPC). It constitutes the chemical index of WCO degradation by measuring the cumulative degradation products. The TPC values were measured with the fast portable device OS280—Cooking Oil Tester (Hanon Advanced Technology Group Co., Ltd., Jinan, China). To determine the TCP, the oil was heated to temperatures between 150 and 180 °C, and the sensor of the device was immersed in the analyzed oil; the value displayed on the instrument screen was recorded. The determinations were made in triplicate.

2.3. Description and Use of Natural Adsorbents

As natural adsorbents, clinoptinolite-type zeolite obtained from volcanic rocks (Perșani Mountain from the Eastern Carpathians), which originates from Zeolites Production Company—Rupea (Brașov County, Romania), and eggshell from ISA Brown hens procured from the “1 Decembrie 1918” University from Alba Iulia, Romania cafeteria, were used as natural adsorbents.
For the chemical method of oil purification, bulk industrial mine salt from Ocna Dej Saltmine, Cluj County, Romania was used.

2.3.1. Preparation of Zeolite

Natural zeolite was washed with distilled water to remove impurities and, then, dried in an oven at 105 °C until constant mass. The zeolite activation was achieved by two methods: thermal and chemical. The thermal activation was accomplished by calcining the natural zeolite at a temperature of 600 °C for 4 h. Chemical activation was performed using 1 M NaOH (Merck, Darmstadt, Germany) and 1 M NaCl (Merck, Darmstadt, Germany). The zeolite with the NaOH or NaCl solution was stirred continuously for 4–6 h; then, it was repeatedly washed with distilled water until the pH of the washing water was neutral. The granulation of the zeolite used was 0.65–1 mm.
In order to determine how well natural zeolite reduced the amount of AV, PV, and TPC from WCO—sunflower oil, various doses of natural zeolite were mixed with the residual oil. Natural, activated, and non-activated zeolite was used in doses of 5, 10, 20, and 30 g/100 mL of WCO. The contact time was up to 120 min, and the sampling was carried out at 15 min intervals. The experiments were carried out under continuous stirring for 120 min at 300 rpm and 80 °C. To check the influence of temperature, a determination was made at 25 °C [35].

2.3.2. Preparation of Eggshell

The eggshell was washed with distilled water to remove impurities and, then, dried in an oven at 105 °C until constant mass. After drying, the eggshell was crushed in a mortar and sieved in a mesh of 42 Tyler. The eggshell used was both thermally and chemically activated. Thermal activation was achieved by calcining the eggshell at a temperature of 900 °C for 4 h. Chemical activation was achieved by mixing the eggshell with a 0.1 M HCl solution (Chempur, Piekary Śląskie, Poland) for 48 h, followed by washing with distilled water to neutral pH.
To test the efficiency in the purification of WCO, different doses of thermally and chemically treated eggshell were tested. Eggshell was used in doses of 5, 10, 20, and 30 g/100 mL of WCO—sunflower oil. The contact time was up to 120 min, and the sampling was carried out at 15 min intervals. The experiments were carried out under continuous stirring at 300 rpm at 80 °C [36].

2.3.3. Purification of WCO Using Saline Solution

The WCO—sunflower and palm oil (200 mL) was heated to 90 ± 5 °C and washed with a hot brine solution (20 mL) [5% and 10% salt mine w/v, respectively], in the ratio of 10:1 (v/v) WCO/brine solution. The hot mixture (WCO and brine solution) was mixed using a stirrer at a speed of 60 rpm for 60 min. Then, the mixture was allowed to settle for 8 h to separate the two phases: the oil and the aqueous phase. The lower phase (brine solution + impurities) was separated. The color of the purified vegetable oil was estimated visually.
The WCO—sunflower and palm oil resulting from the frying of various food products was treated with 5% and 10% saline solution. After the treatment, the variation in the different physicochemical parameters was monitored: moisture, color, viscosity, pH, density, RI, AV, SV, IV, and PV. To evaluate the efficiency of the purification process with saline solution, the values obtained were compared with the values of the untreated WCO and also with the values of the fresh oil [37].

2.4. Methods Used for Characterization of the Natural Adsorbents and Oils

The morphological features of the two modified-zeolite samples and of the eggshells before contact with the cooking oils were determined by scanning electron microscopy (SEM), using a tungsten filament at 15 kV acceleration voltage and a working distance of about 9.5 mm. The elemental chemical composition of the eggshells was estimated using an energy-dispersive X-ray spectrometry (EDX). A TM4000plus II scanning electron microscope (Hitachi, Tokyo, Japan), coupled with a liquid-nitrogen-free EDS mapping detector (Oxford Instruments, Oxford, UK), was used for these purposes.
The chemical composition of the zeolite was determined by X-ray fluorescence spectrometry using a Quant’X ARL energy-dispersive X-ray fluorescence spectrometer (Thermo Fisher Scientific, Waltham, MA, USA).
The chemical functional groups of the oils were examined using FT-IR analysis. Two mL of oil sample at room temperature was used for analysis. A Bruker Vertex 70 spectrometer (Bruker Optik GmbH, Rosenheim, Germany) equipped with Platinium ATR, Bruker Diamond Tip A225/Q.1., at room temperature (4.000–400 cm−1) with a nominal resolution of 4 cm−1 with 128 scans was used. All samples were measured in duplicate.

3. Results

3.1. Simulation of the Thermal Degradation Process of Sunflower Oil

The PV variation in sunflower oil, used in the process of frying lean chicken breast and potatoes, was observed for 60 min at temperatures ranging from 65 °C to 180 °C. In all of the examined samples, the PV value increased over time independent of the temperature at which the sunflower oil was exposed. In general, the rate of the oxidation reaction increases with temperature. As the temperature increases, changes in the partial pressure of oxygen have less influence on the rate of the oxidation reaction because oxygen becomes less soluble in lipids and water.
PV increase is slow at 65 °C but more accelerated after 20 min of heat treatment at temperatures above 90 °C. Starting from a value of 1.20 mEqO2/kg, after 60 min, PV reaches 40.50 mEqO2/kg in the case of treatment at 180 °C and only 10.38 mEq O2/kg in the case of treatment at 65 °C. PV increases with increase in the temperature to which the vegetable sunflower oil is subjected, as well as with time. The heating time significantly influences PVs. Thus, it can be observed that by heating the oil at a temperature of 180 °C for 30 min, the PV is 23.13 mEqO2/kg, and after 1 h, it nearly doubles.
In chemistry, it is uncommon for the studied process to be influenced by a single variable. It is important to observe the cumulative and simultaneous influence of other parameters that, through their variations, notably impact the observed outcome [38].
By using the MATLAB program, the experimental data were processed and analyzed, regarding the time variation in the PV for the sunflower oil samples at different temperatures. In Figure 1, the PVs resulting from the simulation of sunflower oil degradation at different temperatures in time are presented comparatively. The experimental data and the surfaces generated by the statistical mathematical models are presented.
A second-degree polynomial equation with two independent variables was proposed, describing the correlation of the PV of sunflower oil, z, with the frying time, x, and the applied temperature, y.
z = − 6.0948 − 0.1938 · x + 0.1029 · y + 0.0004 · x 2 − 0.0003 · y 2
The accuracy of the correlation was confirmed by the adequacy indicators: the model accuracy indicator (r2 = 0.952), the correlation coefficient (r = 0.976), and the standard deviation (σ = 7.272).
In general, the rate of oxidation increases with temperature. As the temperature increases, changes in the partial pressure of oxygen have less influence on the rate of the oxidation reaction because oxygen becomes less soluble in lipids and water.

3.2. Characterization of Adsorbents Used in the Purification of WCOs

SEM-EDX Analysis

Morphology plays an important role in catalyst activity and selectivity. Figure 2 presents SEM micrographs of the eggshells and zeolite samples activated with NaOH and NaCl at different magnifications.
The chemical composition of the natural zeolite estimated by X-ray fluorescence spectroscopy is presented in Table 2, together with the values of several physicochemical parameters provided by the zeolite supplier in the technical data sheet [39].
The mineralogical analysis of the zeolite revealed that it mainly consists of clinoptilolite (71–83.3%), volcanic glass (4.1–9.7%), plagioclase (6.67%), SiO2 (2.25–2.6%), and traces of other minerals [39].
The elemental chemical composition of eggshell samples was estimated by EDX analysis that was performed on two regions of the surface (corresponding to the regions indicated in Figure 2c: whiter region—spectrum 1; darker region—spectrum 2) with different morphological characteristics as shown in Figure 2c.
EDX analysis of the sample showed that calcium (Ca), oxygen (O), and carbon (C) exhibited the highest concentrations. The whiter region (spectrum 1 in Supplementary Figure S1) was rich in inorganic material, containing calcium (31.18 wt.%), oxygen (50.16 wt.%), and carbon (18.17 wt.%), that corresponds most probably to calcium carbonate [40,41]. Additionally, traces of magnesium were identified. It can be present in the form of magnesium carbonate (MgCO3) [42].
The darker region corresponding to the fibrils of the eggshell membrane (spectrum 2 in Supplementary Figure S1) consisted mainly of organic material, namely, proteins (most likely collagen), containing large amounts of carbon (57.87 wt.%), oxygen (20.13 wt.%), and nitrogen (17.26 wt.%). In addition, small amounts of S (2.56 wt.%), Ca (1.89 wt.%), Si (0.28 wt.%), and Mg (0.05 wt.%) were also detected.
This finding aligns with previous investigations using EDX mapping analysis, which indicated the predominance of calcium carbonate (CaCO3), constituting the main component of the total elemental content [43].

3.3. Purification of WCO Using Saline Solution

3.3.1. Purification of Sunflower—WCO Oil from Food Frying

The WCO—sunflower oil used to fry a food mix of animal and non-animal foods was treated with a saline solution of 5% and 10% in order to improve the physicochemical properties. The obtained results are presented, before and after the application of the purification treatment, in Table 3.
The sunflower oil samples submitted to the frying of a mix of animal and non-animal foods (Table 3) have the same moisture content before and after the treatments applied under normal storage conditions. The color of the samples varies from light yellow (for the fresh oil) to darker colors for the WCO and the purified ones, respectively. Regarding the viscosity of the samples, it increases from 33.72 mm2/s in the case of fresh oil to 49.47 mm2/s in the case of WCO. A slight decrease in the viscosity of the purified samples is observed, and the lowest value of 42.27 mm2/s is recorded for the oil treated with 10% NaCl solution. The pH value decreases from 7.68 (in the case of fresh oil) to 5.71 after frying. Purification treatments with saline solution bring the pH to around 6. The density of sunflower oil increases slightly after frying but decreases after it is purified. The same evolution is observed in the case of the RI parameter.
In the case of the sunflower oil in which the potatoes—variety Roclas (National Institute of Research and Development for Potato and Sugar Beet from Braşov, Romania) were fried, as well as the chicken meat, similar values are recorded as in the case of the oil in which a mix of animal and non-animal products were processed. Although the trend of the physicochemical parameters is similar, differences in color and viscosity are visible.

3.3.2. Purification of Palm—WCO from Food Frying

The values of the physicochemical parameters determined on fresh palm oil, palm—WCO, and purified palm oil subjected to frying of products of non-animal origin (bakery products and a mix of animal and non-animal foods) are presented in Table 4.
The physicochemical results can indicate any chemical changes in the used and purified palm oil. By frying, the values of the main parameters such as moisture content, color, density, viscosity, PV, and IV register significant changes. Through the process of purification, these indicators for evaluating the deterioration of recycled palm oil change favorably both in the case of frying bakery products and food mixes.

3.4. Purification of Sunflower—WCO Using Natural Zeolite

Adsorbents have the ability to reduce the AV and PV in WCO [44]. The values of the chemical parameters before and after 120 min of treatment with different doses of zeolites of 5, 10, 20, and 30 g per 100 mL of WCO are shown in Table 5. To evaluate the influence of temperature on the adsorption process, experiments were performed at temperatures of 25 and 80 °C. Supplementary Table S1 shows the results of the parameters obtained at 15 min intervals for 2 h for each zeolite dose.
For the dose of 5 g of zeolite applied, slight decreases in the acidity of the WCO were noticed. Thus, from the initial value of 3.4 mg KOH/g, the lowest values were recorded for the zeolite activated at 80 °C with NaOH (2.34 mg KOH/g after 60 min) and NaCl (2.39 mg KOH/g after 120 min). In the case of PV, after the adsorption process of 120 min using thermally activated zeolite, the results were comparably better, and the yield was 60.4% at 25 °C and 63.1% at 80 °C working temperature. Regarding the total polar compounds, the removal efficiency was below 2% in both cases.
The same trend was observed in the case of applying a dose of 10 g of zeolite for all the investigated parameters, but the yields obtained were higher. The zeolite activated at 80 °C with NaOH had the greatest reduction in acidity (2.18 mg KOH/g after 120 min). The highest decrease of PV was at a temperature of 25 °C. The lowest value reached was 11.28 mEqO2/kg.
In the case of the 20 g dose, the best values were obtained by treating the waste oil with NaOH-activated zeolite. The value of the PV decreased to 9.04 mEqO2/kg after 120 min of contact, and the efficiency obtained was 73%. Basically, the higher the dose of zeolite used, the larger the surface of the adsorbent becomes to allow the adsorption process to take place in the case of WCO, so that impurities and unwanted compounds are reduced.
The higher the dose of zeolite used (30 g zeolite), the lower the acidity, PV, and TCP values. The lowest AV was for the oil treated with zeolite activated with NaOH, and the PV decreased significantly after 120 min, the adsorption efficiency being about 80%.

3.5. Purification of Sunflower—WCO Using Eggshell

Table 6 shows the variation in the quality parameters of sunflower—WCO before and after 120 min of contact with the eggshell in doses between 5 and 30 g/100 mL of WCO. Supplementary Table S2 shows the results of the parameters obtained at 15 min intervals for 2 h for each dose of eggshell.
The experiments were carried out at 80 °C.
The analysis of the obtained results shows that the values of the chemical parameters (AV, PV and TCP) are influenced by the adsorbent dose and the contact time; thus, the IA and IP values decrease with increasing dose and contact time.
The adsorption time and the amount of adsorbent/eggshell influence the AV and PV of the sunflower—WCO. The analysis results show that the greater the adsorbent mass and the longer the adsorption time, the more capable it is of reducing the AV and PV in WCO. In the same context, Narwati et al. [45] showed that the longer the stirring time and the higher the adsorbent mass, the lower the levels of peroxide and FFA.

3.6. FT-IR Spectroscopy of the Analyzed Oils

Figure 3 shows the FT-IR spectrum recorded for eight sunflower oil samples according to Table 1 (S1—fresh, S2—WCO by frying potatoes, S3—WCO by frying chicken meat, S4—WCO by frying a mix of animal and non-animal food, S5—S4 purified with NaCl 10%; S6—S4 purified with zeolite-NaCl, S7—S4 purified with zeolite-NaOH, S8—S4 purified with eggshell).
In the case of sunflower oils, the results show that the band at the regions of 3008 cm−1 (the unsaturated C-H functional group), 2922 cm−1 (the saturated C-H functional group), and 2853 cm−1 are associated with symmetric and asymmetric C-H of alkanes groups (stretching vibrations of C-H in CH2 and CH3 groups, respectively). The band of 1744 cm−1 is because of the presence of C=O stretching vibration of carbonyl groups present in triglycerides and esters.
Bands that appear between 1500 cm−1 and 500 cm−1 represent a complex spectral band due to the complicated stretching and bending in the substance molecule, which is also called a “fingerprint” region [46,47]. The band at 1463 cm−1 region confirms the bending vibrations of CH2 and CH3 aliphatic groups, while the bending of HCH occurs at 1377 cm−1 and CH2 and scissoring at 1463 cm−1, respectively. The bands situated in the region 1237 cm−1, 1160 cm−1, 1097 cm−1 present the stretching vibration of C=O ester group.
Figure 4 presents the FT-IR spectrum recorded for three palm oil samples (S9—fresh oil, S10—WCO by frying bakery and pastry products, S11—S10 purified with NaCl 10%). Since palm oil is semi-solid at room temperature and contains several saturated and unsaturated fats, only the treatment with 10% NaCl solution was analyzed by FT-IR.
A similar trend is also observed in the case of palm oils. The absorption band of the carbonyl group C=O in saturated aliphatic esters usually appears at 1743 cm−1. Stretching vibrations of groups C-H—Methyl (-CH3) and methylene (-CH2-), both symmetrical and non-symmetrical, are present at 2851, 2916, and 3038 cm−1. The C-O stretching vibration at 1377 cm−1 is also observed in the spectra of other organic compounds such as carboxylic acids, ethers or alcohols. Methyl esters belonging to long-chain fatty acids often give rise to absorption maxima at about 1177 cm−1.

4. Discussion

During the process of repeated frying (180 °C) of fats and oils, a large number of volatile and non-volatile substances are formed due to the reactions that occur and which cause changes in the structure of fats and oils [48]. The PV is important for monitoring the peroxides in the first stages of oxidation. The results vary depending on the process used and the temperature [49]. Values of PV around 40.50 mEq O2/kg obtained from the simulation during heating at 180 °C of the sunflower oils are in accordance with other studies [4,50].
The degradation of cooking oil is characterized by changes in color, smell, taste, increase in water content, PV, FFA, and metal contamination. These changes are caused by the process of hydrolysis, oxidation, and polymerization of unsaturated fatty acids and results in the formation of ketones, aldehydes, and polymers [43]. These will generate a disagreeable flavor and also impair certain vitamins and essential fatty acids present in the oil. Oil containing water triggers a hydrolysis reaction that causes the oil to go rancid [51]. The water content of the oil greatly affects the quality of the oil, although the low oil’s FFA content and PV and a high-water content can lead to a high rate of hydrolysis. This process breaks down fats into glycerol and FFA, potentially posing health risks [52].
WCO can be reused safely if it goes through a purification and bleaching process. Purification of WCO using adsorbents is a simple and effective process [53].
Utilizing adsorbents for oil purification can diminish the presence of metal contaminants, water content, and organic and inorganic compounds. Activated carbon can adsorb anions, cations, and molecules in the form of organic and inorganic compounds in solution or gas. The adsorption power is influenced by the porosity of the activated carbon and can be increased by its chemical or physical activation (by heating, which causes the pores of the carbon to open). The use of activated carbon in the purification process is able to adsorb up to 95–97% of the total dyes contained in the oil. Yustinah’s research [54] shows that the use of activated carbon from coconut shells can reduce oil turbidity and that from sawdust can remove color, resulting in a 2.04% increase in the brightness of the color of WCO [55].
The effectiveness of the adsorption process depends on the dose to be used, but also on the contact time [56]. The stirring time is one of the factors that leads to the decrease of the peroxide level, which agrees with findings by Fitriyana and Safitri [57]. They clarified that peroxide, which contains oxygen, is a polar compound that readily binds to polar adsorbents. Eggshells contain minerals, especially calcium carbonate. This material has a porous structure that gives it the ability to adsorb certain impurities and polar compounds from oils [58]. Activated chicken eggshell powder primarily consists of polar calcium carbonate [59]. Stirring enhances the interaction strength between the adsorbate and adsorbent, influenced by their respective polarities. Furthermore, the adsorption process involves van der Waals forces, fostering attraction between molecules.
Increasing the amount of eggshell powder as an adsorbent greatly contributes to increasing the adsorption efficiency. These results coincide with those reported by Tizo et al. [60] and Narwati et al. [45]. The adsorption speed depends on the type and characteristics of the adsorbate, in our case, zeolite or eggshell, and depends on the activation mode. At the same time, the adsorption kinetics are influenced by the expected operating conditions, the temperature, and the dose of the adsorbent.
Zeolites are known for their ability to adsorb various substances from liquid and gaseous media, due to their porous structure and the large surface area they provide. This makes them effective in purifying and treating WCO [61]. Zeolites are crystalline aluminosilicates in which the three components, aluminum, silicon, and oxygen, are organized into a consistent framework, featuring uniform-sized pores and cavities [62]. Natural zeolite contains certain cations such as Ca2+, Na+, K+, Mg2+, a statement supported by the results of chemical analysis. These positive ions can be readily exchanged for negative ions in a solution [63]. The use of zeolites in the removal of polar compounds from WCO involves their addition for the purpose of adsorbing polar compounds such as water, FFA, and other water-soluble impurities. After that, the impurity-laden zeolites can be separated from the treated oil, usually by filtration or other separation techniques [64,65,66]. The effectiveness of the process depends on the type of zeolite used, the specific operating conditions, and the exact composition of the WCO.
The Si/Al ratio, or ratio of tetravalent (Si, Ge, etc.) to trivalent (Al, B, etc.) atoms, significantly influences the polarity of the material. For instance, zeolites with a higher Al content, also referred to as low-silica zeolites, exhibit greater affinity for polar compounds, such as water, compared to high- or pure-silica zeolites [67]. The higher the dose of zeolite added, the lower the value of the PV in the oil, thus increasing the quality of the purified oil [68]. Consequently, our results also indicate that the AV, PV, and TPC values decrease with increase in the dose of zeolite added to the sunflower—WCO. Our results show that both zeolite and eggshell can reduce the acidity of residual oil. Natural zeolite from Rupea-Brașov, Romania, was used for the first time as an absorbent to reduce the AV and PV of WCO. The results are comparable to those in the literature [23,35]. The hydrolysis process of cooking oil is expedited by the presence of water within the oil [69]. RI is a valuable parameter to assess the degree of oil unsaturation. As the number of conjugated double bonds in the oil increase, the RI correspondingly increases [70]. Ghaly and El-khamissi [71] demonstrated that gradual increases in AV are noticeable for sunflower oils with increasing frying time. Similarly, in our result, the AV increases in the case of fresh sunflower oil from 0.73 mgKOH/g to 6.4 mgKOH/g in the case of the food mix, 9.4 mgKOH/g in the case of the oil in which potatoes were fried, and 11.41 mgKOH/g for the oil in which the chicken meat was thermally treated. In addition, the same trend is observed in the case of palm oil, where an increase from 1.39 mgKOH/g to 4.66 mgKOH/g is recorded for the oil in which bakery products were fried and 6.62 mgKOH/g in the case of frying the food mix. Furthermore, earlier studies [72] have shown a rise in AV if the number of frying cycles increases.
SV represents the average molecular weight or chain length of the fatty acids contained in an oil, primarily in the form of triglycerides [73]. SV was 193.01 mg KOH/g oil in sunflower fresh oil and 188.38 mg KOH/g in palm fresh oil and between 198.94 and 213.03 mg KOH/g oil in sunflower—WCO and between 210.69 and 213.03 mg KOH/g oil for palm—WCO, similar to values obtained by Abiodun et al. [74]. The higher value of SV, however, suggests that the mean molecular weight of fatty acid is higher or that the number of ester bonds is more. This might imply that the fat molecules do interact with each other. A higher SV indicates a greater mean molecular weight of fatty acids or a higher number of ester bonds. This observation suggests a potential interaction among the fat molecules [75]. The degradation of vegetable oils is evidenced by the decrease in IV. IV is used to quantify unsaturation or the average number of double bonds in fats and oils [76].
IV is a measure of the degree of unsaturation of fatty acids present in fat and oil [77]. The greater the IV of an oil, the higher its level of unsaturation. The IV decreases from 129.02 g I2/100 g in the case of fresh sunflower oil to values between 116.14 and 122.14 g I2/100 g for the used one. The values for palm oil decrease from 55.5 g I2/100 g in the case of the fresh one to 43.20 and 45.72 g I2/100 g for the one used for frying and are consistent with the studies undertaken by Kabutey et al. [78]. The viscosity of oil increases with the length of fatty acids and the formation of polymers [79]. The PV of oils serves as a measure of its freshness by measuring the primary oxidative damage of edible oils, namely, the conversion of hydroxyl groups of unsaturated fats in oils by molecular oxygen to hydroperoxides and peroxides [80]. The results of the research show that the effects of treating sunflower—WCO with zeolite are similar to those of Petrović et al. [81], with natural adsorbents showing high efficiency in reducing the PV.
Through heat treatment applied to food cooking, oils lose unsaturation from radical reaction (insertion, addition, substitution, bond breaking, and bond formation) with atmospheric oxygen [82]. In the case of the studied sunflower and palm oils, it was found that the spectra of the WCO (S2, S3, S4, S10) and the purified (S5–S8, S11) and fresh oils (S1 and S9) were very similar to each other.
This results from the fact that the functional groups of the WCO samples remained intact after the application of heat treatment and different foods. The degree of unsaturation of the oil is analyzed based on some characteristics of the frequency bands—FT-IR. The percentage of transmittance for all the functional group regions show a small increment from fresh cooking oil (S1 and S9) in comparison with WCO (S2–S4 and S9) and the purified ones (S5–S8 and S11). These provide information on the quality of the oils and a unique mechanism for the deterioration of the oil samples.
The FT-IR spectra of palm oil are largely similar to those of sunflower oils subjected to frying treatments, and the greatest difference is observed in the range of 500–1500 cm−1.
The bands at 1743 cm−1 for both sunflower and palm oil lies in the range 1800–1700 cm−1, which can be attributed to C=O stretching, typical of esters, and, thus, are common in both fatty acid methyl esters and refined oil spectra. These findings are similar to tose of Omidvarborna et al. [83] and Shalaby and El-Gendy [84]. FT-IR bands between 1100 and 1300 cm−1 can be attributed to the stretching of C-O and are found in the spectra of other organic compounds such as carboxylic acids, ethers, or alcohols. Methyl esters of long-chain fatty acids lead to bands of 1237, 1160, and 1097 cm−1. This observation is in line with those of Parada Hernandez et al. [85] and Rabelo et al. [86]. The band of 1377 cm−1 can be attributed to the glycerol group O-CH2 (mono-, di-, and triglycerides) aligning with the findings of Dube et al. [87]. According to the results, the bands of 2922 and 2853 cm−1 show the stretching vibrations of C-H in CH2 and CH3 groups, respectively. Ali et al. [88] evaluated FT-IR spectra of biodiesel produced from WCO. Vlachos et al. [89] explained that the maximum absorption of the infrared at 3006 cm−1 band in the case of palm oil is related to the unsaturation degree of the vegetable oils. Poiana et al. [90] showed the height of the band increases with the degree of alteration of the oils. In the case of sunflower oil, the band at 3008 cm−1 which is in the range of 3007–3009 cm−1 corresponds to C-H symmetric stretching vibration of the cis double bonds, =CH [89,91]. Applications of purified oils are related to environmental pollution, through the production of biodiesel, animal feed, or in their use in the cosmetic industry (detergents, soaps, etc.) [14,37,92]. Studies have shown the efficiency of eggshells as a catalyst in the preparation of biodiesel from waste oil. The absorptive properties of eggshells are very little investigated; therefore, starting from the catalytic properties of eggshells in reducing waste oil acidity, in the present work, the absorptive capacity of eggshells in reducing waste oil acidity was tested [36,93,94].

5. Conclusions

This study aimed to simulate the sunflower oil frying process with PV monitoring. By using the statistical mathematical model’s equation, we were able to correlate the variation in PV with temperature and time. From the comparative analysis of the main physicochemical parameters of fresh, used, and purified sunflower and palm oils, it was found that zeolite activated with NaCl adsorbent has a more efficient yield than eggshell. In the case of sunflower oil, it was found that the adsorption time and the amount of adsorbent/eggshell influences the AV and PV of the WCO. Regarding AV, zeolite activated with NaOH and heat-treated eggshells, at the dose of 30 g, had the highest purification efficiency. In the case of PV, zeolite had a much higher efficiency, the value obtained after 120 min being 7.81 mEqO2/kg, compared to that of using eggshells, which was 20.33 mEqO2/kg. Zeolite was proved to be an effective material in oil recycling operations because of its adsorption qualities, which enable pollutants and impurities to be retained in oils. The potential of the adsorbent (zeolite or eggshell) to reduce the AV and PV of WCO increases with its dose and adsorption period. Natural zeolite from Rupea-Brașov, Romania, was used for the first time as an adsorbent in the purification of WCO.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/app14114417/s1, Figure S1: EDX spectra of the chicken eggshell composition corresponding to the regions indicated in Figure 2c: whiter region—spectrum 1; darker region—spectrum 2; Table S1: The values of the quality parameters of the sunflower-WCO treated with natural zeolite; Table S2: The values of the quality parameters of the sunflower WCO treated with eggshell.

Author Contributions

Conceptualization, M.G. and R.B.; methodology, M.G.; software, G.-A.D.; validation, M.G., S.V. and G.I.R.; formal analysis, R.B., S.V. and G.I.R.; investigation, M.L.V. and G.-A.D.; resources, R.B.; data curation, S.V. and I.G.; writing—original draft preparation, M.G. and I.G.; writing—review and editing, M.P.; visualization, M.L.V.; supervision, M.P.; project administration, M.P.; funding acquisition, R.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Acknowledgments

We are thankful to Zeolites Production Company, from Rupea, Brasov County, Romania for the generous donation of zeolite. Thank you for the technical support of “1 Decembrie 1918” University of Alba Iulia cafeteria, for the eggshells and to the university student volunteers who helped with the heavy workload in analyzing the oils.

Conflicts of Interest

The authors declare no conflicts of interest.

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Applsci 14 04417 g001
Figure 1. PV variation in sunflower oils depending on the applied temperature and frying time.
Figure 1. PV variation in sunflower oils depending on the applied temperature and frying time.
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Figure 2. SEM micrographs of the zeolite samples activated with NaOH (a) and NaCl (b) and of the eggshells (c).
Figure 2. SEM micrographs of the zeolite samples activated with NaOH (a) and NaCl (b) and of the eggshells (c).
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Figure 3. FT-IR spectra of fresh sunflower oil (S1), WCO (S2, S3 and S4), and purified oils (S5, S6, S7 and S8).
Figure 3. FT-IR spectra of fresh sunflower oil (S1), WCO (S2, S3 and S4), and purified oils (S5, S6, S7 and S8).
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Figure 4. FT-IR spectra of fresh palm oil (S9), WCO (S10), and purified oil (S11).
Figure 4. FT-IR spectra of fresh palm oil (S9), WCO (S10), and purified oil (S11).
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Table 1. The vegetable oil samples used and the treatments applied to them.
Table 1. The vegetable oil samples used and the treatments applied to them.
Sample No.Type of Vegetable OilNote
S1Fresh sunflower oil-
S2WCO—sunflower oil from frying potatoes20 g of sliced potatoes in 100 mL of oil
S3WCO—Sunflower oil from frying chicken20 g of lean chicken meat in 100 mL of oil
S4WCO—Sunflower oil from frying a mix of potatoes and chicken10 g of sliced potatoes and 10 g of lean chicken meat in 100 mL of oil.
S5S4 treated with 10% NaCl sol.-
S6S4 treated with zeolites activated with NaCl-
S7S4 treated with 30 g zeolite activated with NaOH/120 min-
S8S4 treated with 30 g eggshell heat-treated/120 min-
S9Fresh palm oil-
S10WCO—palm oil from frying bakery products20 g of doughnuts in 100 mL of oil
S11S10 treated with 10% NaCl sol.-
Table 2. Chemical composition and several physicochemical parameters of the zeolite.
Table 2. Chemical composition and several physicochemical parameters of the zeolite.
Compoundwt. %Other Parameters 1Value
SiO270.93Cation-exchange capacity
(CEC)		1.51 mqv/100 g
Al2O316.21
CaO4.72Specific surface area (BET)23.4 m2/g
K2O3.69Apparent specific gravity (AG)1.65–1.75
Fe2O32.82Total porosity (TP)33.08%
MgO0.46Water absorption16.21%
Na2O0.45Bulk density0.88 kg/dm3
TiO20.25--
BaO0.10--
MnO0.05--
Loss on ignition (LOI)0.21--
1 Parameters from the Product Specification Sheet of the zeolite from Rupea, Romania.
Table 3. Parameters of sunflower oil subjected to the frying of a food mix, sliced potatoes and lean chicken meat.
Table 3. Parameters of sunflower oil subjected to the frying of a food mix, sliced potatoes and lean chicken meat.
Oil TypeMoisture, %ColorViscosity, mm2/spH at 25 °CDensity, g/cm3RI at 25 °C, nDAV, mg KOH/gSV, mg KOH/gIV, g I2/100 gPV, meq O2/kg
Fresh2.50 ± 0.21Clear—pale yellow33.72 ± 0.037.68 ± 0.050.82 ± 0.101.4683 ± 0.000200.73 ± 0.06193. 01 ± 1.35129.02 ± 1.241.20 ± 0.08
Frying
Potatoes + chicken		WCO2.90 ± 0.62Dark brown49.47 ± 0.055.71 ± 0.080.91 ± 0.061.4751 ± 0.000126.40 ± 0.10208.18 ± 0.40116.14 ± 1.3440.50 ± 0.27
NaCl 5%2.22 ± 0.23Light brown43.44 ± 0.026.23 ± 0.250.60 ± 0.081.4702 ± 0.000112.17 ± 0.08198.32 ± 0.42117.51 ± 2.225.43 ± 0.35
NaCl 10%2.38 ± 0.16Brown42.27 ± 0.046.41 ± 0.190.64 ± 0.051.4700 ± 0.0001042.00 ± 0.13196.32 ± 0.66116.94 ± 2.365.31 ± 0.14
Frying
potatoes		WCO2.37 ± 1.08Dark brown47.56 ± 0.024.96 ± 0.090.90 ± 0.061.4720 ± 0.000119.41 ± 0.19204.08 ± 1.10122.14 ± 0.3444.43 ± 0.27
NaCl 5%2. 20 ± 0.36Light brown44.72 ± 0.035.68 ± 0.120.60 ± 0.131.47098 ± 0.000052.57 ± 0.62193.16 ± 2.45118.52 ± 2.969.53 ± 0.74
NaCl 10%2.83 ± 0.66Brown43.68 ± 0.055.93 ± 0.080.69 ± 0.051.47092 ± 0.000082.45 ± 0.45194.22 ± 1.32120.18 ± 2.0510.06 ± 0.38
Frying chickenWCO2.89 ± 0.44Dark brown4.72 ± 0.0946.95 ± 0.060.87 ± 0.061.4755 ± 0.001211.41 ± 0.19198.94 ± 0.72121.14 ± 2.3446.02 ± 0.27
NaCl 5%2.67 ± 0.61Light brown6.27 ± 0.1143.75 ± 0.040.72 ± 0.121.4706 ± 0.000072.78 ± 0.33195.25 ± 1.32119 ± 1.097.50 ± 0.19
NaCl 10%2.89 ± 0.83Light brown6.33 ± 0.0742.91 ± 0.030.66 ± 0.081.4708 ± 0.000092.70 ± 0.12195.98 ± 0.88120 ± 2.448.44 ± 0.26
All values are means ± standard deviations (n = 3).
Table 4. Parameters of palm oil subjected to frying bakery products and a food mix.
Table 4. Parameters of palm oil subjected to frying bakery products and a food mix.
Oil TypeMoisture, %ColorViscosity, mm2/spH at 25 °CDensity, g/cm3RI at 25 °C, nDAV, mg KOH/gSV, mg KOH/gIV, g I2/100 gPV, meq O2/kg
Fresh0.21 ± 0.18Pale yellow30.56 ± 0.026.47 ± 0.050.70 ± 0.091.45 ± 0.461.39 ± 0.17188.38 ± 0.4655.5 ± 1.351.20 ± 0.08
Frying
bakery products		WCO0.82 ± 0.09Dark yellow35.08 ± 0.035.03 ± 0.100.77 ± 0.041.42 ± 0.104.66 ± 0.11210.69 ± 1.0343.20 ± 0.5246.02 ± 0.27
NaCl 5%0.46 ± 0.12Light yellow33.24 ± 0.015.40 ± 0.290.61 ± 0.161.39 ± 0.242.54 ± 0.30199.0 ± 1.4244.26 ± 2.467.50 ± 0.19
NaCl 10%0.52 ± 0.23Light yellow33.47 ± 0.034.30 ± 0.140.63 ± 0.1113.40 ± 0.302.45 ± 0.27185.3 ± 1.6645.07 ± 2.108.44 ± 0.26
Frying
potatoes+ chicken		WCO0.95 ± 0.24Dark yellow37.2 ± 0.044.82 ± 0.060.80 ± 0.081.44 ± 0.126.62 ± 0.27213.03 ± 2.2645.72 ± 0.55182.20 ± 3.33
NaCl 5%0.63 ± 0.16Yellow35.15 ± 0.035.90 ± 0.100.61 ± 0.171.43 ± 0.334.84 ± 0.44200.18 ± 1.5746.41 ± 0.36158.00 ± 2.04
NaCl 10%0.55 ± 0.20Light yellow34.21 ± 0.065.82 ± 0.700.71 ± 0.091.40 ± 0.135.06 ± 0.32178.61 ± 3.0343.72 ± 1.85154.18 ± 2.87
All values are means ± standard deviations (n = 3).
Table 5. The values of the quality parameters of the sunflower—WCO treated with natural zeolite.
Table 5. The values of the quality parameters of the sunflower—WCO treated with natural zeolite.
Zeolite Dosage, gTime, minAV, mg KOH/gPV, mEqO2/kgTPC, %
25 °C80 °C25 °C80 °C25 °C80 °C
ZT 1ZT 1ZNaOH 2ZNaCl 3ZT 1ZT 1ZT 1ZT 1
03.40 ± 0.1040.5 ± 0.8243.0 ± 0.01
51202.71 ± 0.072.50 ± 0.092.36 ± 0.082.39 ± 0.1016.04 ± 1.3714.93 ± 0.8842.8 ± 0.1142.2 ± 0.13
102.75 ± 0.072.64 ± 0.022.18 ± 0.032.30 ± 0.0611.28 ± 0.2312.51 ± 0.1842.4 ± 0.1042.0 ± 0.12
202.63 ± 0.052.06 ± 0.031.93 ± 0.062.44 ± 0.059.04 ± 0.8310.93 ± 0.9142.2 ± 0.1341.9 ± 0.11
302.70 ± 0.092.03 ± 0.031.16 ± 0.072.07 ± 0.047.81 ± 0.447.93 ± 0.3042.0 ± 0.1041.7 ± 0.09
1 ZT—Thermally activated; 2 ZNaOH—Activated with NaOH; 3 ZNaCl—Activated with NaCl; All values are means ± standard deviations (n = 3).
Table 6. The values of the quality parameters of the sunflower—WCO treated with eggshell.
Table 6. The values of the quality parameters of the sunflower—WCO treated with eggshell.
Eggshell Dosage, gTime,
min		AV, mg KOH/gPV, mEqO2/kgTPC, %
EST 1ESTHCl 2EST 1ESTHCl 2EST 1ESTHCl 2
03.40 ± 0.1040.5 ± 0.8243.0 ± 0.01
51201.51 ± 0.262.04 ± 0.1733.61 ± 0.4433.84 ± 0.2343.0 ± 0.0443.0 ± 0.02
101.77 ± 0.051.95 ± 0.0632.62 ± 1.4324.52 ± 0.8143.0 ± 0.0243.0 ± 0.08
201.57 ± 0.081.85 ± 0.0329.75 ± 1.6830.65 ± 1.5643.0 ± 0.0643.0 ± 0.03
301.10 ± 0.041.44 ± 0.0620.82 ± 1.2720.33 ± 1.1443.0 ± 0.0143.0 ± 0.03
1 EST—Thermally activated, 2 EGHCl—Activated with HCl; All values are means ± standard deviations (n = 3).
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Bostan, R.; Glevitzky, M.; Varvara, S.; Dumitrel, G.-A.; Rusu, G.I.; Popa, M.; Glevitzky, I.; Vică, M.L. Utilization of Natural Adsorbents in the Purification of Used Sunflower and Palm Cooking Oils. Appl. Sci. 2024, 14, 4417. https://doi.org/10.3390/app14114417

AMA Style

Bostan R, Glevitzky M, Varvara S, Dumitrel G-A, Rusu GI, Popa M, Glevitzky I, Vică ML. Utilization of Natural Adsorbents in the Purification of Used Sunflower and Palm Cooking Oils. Applied Sciences. 2024; 14(11):4417. https://doi.org/10.3390/app14114417

Chicago/Turabian Style

Bostan, Roxana, Mirel Glevitzky, Simona Varvara, Gabriela-Alina Dumitrel, Gerlinde Iuliana Rusu, Maria Popa, Ioana Glevitzky, and Mihaela Laura Vică. 2024. "Utilization of Natural Adsorbents in the Purification of Used Sunflower and Palm Cooking Oils" Applied Sciences 14, no. 11: 4417. https://doi.org/10.3390/app14114417

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