Waste-Valorized Biodiesel from Avocado Seed Oil: Production, Blending and Fuel-Property Characterization

INTRODUCTION

Rapid increase in global population, urbanisation and industrialization has all increased energy demands primarily met by fossil fuels, leading to pollution menace [1]. Over the years, fossil fuel dominance has raised concerns about environmental degradation, non-renewability and health risks, prompting developed nations and researchers to seek renewable, eco-friendly alternatives with minimal health impacts [2]. Fossil fuel pollution causes many serious effects on human health; for instance, particulate matters (PM), nitrogen oxides (NO_x), unburned hydrocarbons, and carbon monoxide released from petroleum and coal operations can cause lung cancer, fetal harm and increased morbidity [3].  Among renewable energy sources like solar, wind, and biofuels (bioethanol, biodiesels), biofuels obtained from vegetable oils, animal fats, and used cooking oils offer viable substitutes due to their renewability [4][5][6]. Notably, biodiesel, an example of a biofuel is obtained from renewable resources such as plants and animals [7]. Biodiesel, especially monalkyl esters from renewable feedstocks for diesel engines, provides advantages over petroleum diesel, including sustainability, non-toxic emissions, biodegradability, high cetane numbers for smoother operation, ease of production, lower health risks, superior flash points(>100°C) for safer handling, and blend compatibility [8-16]. Compared to petro-diesel, biodiesel exhibits favourable combustion profiles with reduced CO, particulate matter, and hydrocarbon emissions [7]. Prevalent process of generating biodiesel is via transesterification which implies to a catalyzed chemical reaction involving the use of vegetable oil and an alcohol to yield fatty acid alkyl esters and glycerol [17]. Dominant biodiesel generation method, transesterification (alcoholysis) involves the relocation of alcohol from ester with a different alcohol [18]. Research showed that production of biodiesel by transesterification includes the use of alcohols, especially C₁ and C₂ chain alcohol (methanol and ethanol), in a chemical reaction with animal fats or vegetable oil in the presence of alkaline condition to yield long chain of fatty acid methyl ester [19]. Catalysts used in transesterification may either be alkaline, acid or enzyme materials. For alkaline catalysts, the predominantly used substances are sodium hydroxide (NaOH) and potassium hydroxide (KOH) [17]. Avocado (Persea americana), a tropical Lauraceae species grown in Central/ South America, West Indies, the Caribbean, and African nations including Nigeria Cameroon and Ghana produce seeds containing biodiesel-convertible triglycerides. [20]. Studies confirm optimal yields from Hass avocado seeds at 6:1 oil: alcohol ratios while heterogeneous H₂ SO₄ / KOH catalysis also achieved high yields with physicochemical properties meeting ASTM standards [17][18] This study extracted oil from avocado seeds via Soxhlet-n-hexane extraction, produced B100 biodiesel through alkali-catalysed transesterification, blended as B10 and B20 with petroleum diesel, and evaluated properties against ASTM D6751/EN 14214 standards to validate their potential as sustainable petro-diesel substitutes.

MATERIALS AND METHOD

Sample collections and preparation

The material (avocado seeds) used for this research were gathered from a farm in Awka, Anambra state, Nigeria. The avocado seeds were washed thoroughly to eradicate dirt and sun dried for two weeks in order to remove the water content of the seeds. The sundried seeds were cut into pieces, crushed to fine powder and corked in plastic bottles prior to all the experiments.

Extraction of avocado seed oil

About 300 g of the sample was measured into a thimble and loaded into Soxhlet extractor with 250 mL of n-hexane solvent in the extraction flask. Condenser was placed into the Soxhlet extractor. The instrument was insert on a heater and was left to reflux for about 2 h. The method was repeated until the oil was extracted from the avocado seeds. The solvent was recovered using a rotary evaporator [21]

Biodiesel Production

A measured volume of 200 mL of avocado seed oil was preheated to approximately 55 °C to reduce viscosity and facilitate the transesterification reaction. A methoxide solution was prepared by dissolving 1.006 g of potassium hydroxide (KOH) in 20 mL of methanol under continuous stirring until complete dissolution was achieved. The freshly prepared methoxide solution was then gradually introduced into the preheated oil under controlled conditions. The reaction mixture was subjected to reflux at 55 °C for 60 minutes with constant stirring to ensure efficient conversion of triglycerides into fatty acid methyl esters (FAME), commonly referred to as biodiesel. Upon completion of the reaction, the mixture was transferred into a separatory funnel and allowed to settle undisturbed for approximately 8 hours to achieve phase separation. Two distinct layers were formed: the lower glycerol-rich phase and the upper biodiesel phase. The glycerol layer was carefully drained off, and the crude biodiesel was subsequently washed multiple times with warm distilled water to remove residual catalysts, soap, and impurities. The washed biodiesel was then subjected to drying in a hot air oven at around 100 °C to eliminate any residual moisture, yielding purified biodiesel suitable for further analysis.

Characterization of Oil and Biodiesel

The physicochemical properties of both the raw avocado seed oil and the synthesized biodiesel were systematically evaluated following standard analytical procedures. The characterization was carried out to assess the quality, fuel suitability, and compliance with biodiesel standards. Physical properties analyzed included kinematic viscosity, relative density (specific gravity), pour point, cloud point, flash point, fire point, refractive index, and cetane number. These parameters are critical for determining fuel flow behavior, ignition quality, and storage stability under varying environmental conditions. Chemical properties evaluated comprised acid value, iodine value, saponification value, free fatty acid (FFA) content, and peroxide value. These parameters provide insight into the chemical stability, degree of unsaturation, oxidative degradation, and overall quality of the oil and biodiesel. The combined analysis of these properties enables a comprehensive assessment of the efficiency of the transesterification process and the suitability of the produced biodiesel as an alternative renewable fuel.

Determination of Relative Density (Specific Gravity)

The relative density of the oil and biodiesel samples was determined using a clean, dry specific gravity bottle following standard procedures. Initially, the empty specific gravity bottle was weighed using an analytical balance, and the mass was recorded as W1W_1W1​. The bottle was then filled with distilled water at room temperature, ensuring no air bubbles were trapped, and weighed again to obtain W2W_2W2​.

Afterward, the bottle was thoroughly emptied, cleaned, dried, and allowed to cool to room temperature. It was then filled with the test sample (oil or biodiesel), and the mass was recorded as W3W_3W3​.

The relative density (specific gravity) of the sample was calculated using the following expression:

                                                    1

Where,

W₁= weight of empty bottle

W_{2 =} weight of empty bottle + weight of water

W_{3 =} weight of empty bottle + weight of oil

Kinematic Viscosity

Determination of Kinematic Viscosity

The kinematic viscosity of the oil and biodiesel samples was determined using a calibrated viscometer following standard procedures. The sample was carefully introduced into the viscometer tube, which was then immersed in a constant-temperature viscometer bath maintained at 40 °C to ensure thermal equilibrium. Using a suction device, the sample was drawn above the upper calibration mark of the viscometer and then allowed to flow freely under the influence of gravity. The time required for the sample to pass from the upper mark to the lower mark was measured using a digital stopwatch. This procedure was repeated three times to ensure accuracy, and the average flow time was calculated.

The kinematic viscosity of the sample was determined using the following relationship:

Kinematic viscosity (                       2

Where:

KV = Kinematic viscosity

t = time in seconds

K = Viscometer Constant = 0.00768

Flash point and fire point determination

Each of the samples was put into clean crucibles and mercury thermometers inserted in the centre of the samples. Heat was applied to the samples. Flame was subsequently passed over the samples at an interval until a point when a flash was noticed on the samples. The temperature at which it happened was the flash point and the subsequent temperature at which the samples ignited fire for at least 10 seconds was the fire point.

Refractive index

 To achieve this, some drops of the samples were put on the measuring prism and closed with the cover plate. The refractive index was noted through the aperture in % Brix [23].

Determination of Cloud Point and Pour Point

The cloud point and pour point of the oil and biodiesel samples were determined using standard low-temperature evaluation procedures. Each sample was transferred into a clean, dry test tube, which was then placed inside an air jacket containing a cooling medium composed of crushed ice and sodium hydroxide pellets to achieve progressively lower temperatures. The samples were cooled gradually, and the temperature was monitored continuously. At intervals of 5 °C, the test tube was removed briefly and visually inspected under adequate lighting conditions. The cloud point was recorded as the temperature at which the first visible haze or cloudiness appeared in the sample, indicating the onset of wax crystal formation. The pour point was determined as the lowest temperature at which the sample ceased to flow when the test tube was held horizontally for a minimum of five seconds. All measurements were conducted carefully to ensure uniform cooling and accurate observation, as these parameters are critical indicators of low-temperature flow properties and fuel performance under cold conditions.

Cetane number

The Cetane number was computed according to [8].

                      3

Where, CN = Cetane number;

IV = Iodine value;

SV = Saponification value.

Determination of Peroxide Value (PV)

The peroxide value of the oil and biodiesel samples was determined using an iodometric titration method to assess the extent of primary oxidation. Accurately, 10 g of each sample was weighed into a clean, dry conical flask, followed by the addition of 30 mL of an acetic acid–chloroform mixture (3:2 v/v). The flask was gently swirled to ensure complete dissolution of the sample. Subsequently, 0.5 mL of saturated potassium iodide (KI) solution was added, and the mixture was allowed to react in the dark for approximately one minute with intermittent shaking to liberate iodine. After the reaction period, 30 mL of distilled water was added to the mixture. The liberated iodine was titrated against standardized sodium thiosulfate solution with continuous stirring until the yellow color became pale. At this stage, a few drops of freshly prepared starch indicator were added, resulting in a blue coloration. Titration was continued until the blue color disappeared, indicating the endpoint. A blank determination was performed under identical conditions without the sample to correct for reagent impurities.

The peroxide value was calculated using the following expression:

                                   4

Where: T = titration volume for sample;

B = Titration volume for the blank;

N = Normality of thiosulphate used;

g = weight of the sample.

Determination of Saponification Value (SV)

The saponification value of the oil sample was determined using a standard titrimetric method to estimate the average molecular weight of fatty acids present. Accurately, 2 g of the oil sample was weighed into a 250 mL round-bottom flask, and 25 mL of alcoholic potassium hydroxide (KOH) solution was added. The mixture was heated under reflux using an air condenser on a water bath for approximately 30 minutes to ensure complete saponification of the triglycerides. After refluxing, the flask was allowed to cool to room temperature. Subsequently, 2–3 drops of phenolphthalein indicator were added to the mixture. The excess KOH was then titrated against standardized 0.5 N hydrochloric acid (HCl) until the pink color disappeared, indicating the endpoint. A blank determination was performed simultaneously under identical conditions without the oil sample to account for the total amount of KOH initially present.

                            5

Iodine value

About 0.2 g of the sample was measured into 500 mL conical flask. Exactly 20 mL of chloroform was added. The mixture was kept in dark for 1 h. About 20 mL of KI solution was poured into the sample and carefully swirled. It was titrated against 0.1 N Na₂S₂O₃ solution using starch as an indicator with vigorous shaking. A blank was carried out similarly in the absence of the sample.

                                 6

Where:

 A = mL of Na₂S₂O₃;

N = Normality of Na₂S₂O₃.

Free fatty acid analysis

About 2 g of the sample was dissolved in 20 mL ethanol inside a conical flask. Also, 3 drops of phenolphthalein indicator was added to the solution and titrated against 0.1N NaOH. Free fatty acids were computed as follows:

                              7

Where:

V NaOH = Volume of sodium hydroxide used;

N = Normality of sodium hydroxide;

MW = Molecular weight of predominant fatty acid.

Results and discussion

 

 

 

The physical and chemical characteristics of avocado seed oil (ASO) presented in Table 1 confirmed that the seeds are a potent non-consumable, waste-derived feedstock for generation of biodiesel. The orange colour of the oil is likely due to carotenoid -or other pigment component naturally present in the biomass. The oil content of 12.50 % fell within the 15 % range reported elsewhere and is relatively high compared with many biodiesel feedstocks, indicating good economic potential without competing with food- grade oils [17]. The specific gravity (0.901) and viscosity (8.70 mm²/s) of ASO exceed typical diesel values, confirming that the crude oil requires modification before applying in diesel engines. The acid value (3.02 mg KOH/ g) and FFA (1.55 %) are below 2 %, allowing direct base- catalysed transesterification without pre-esterification, while oils with FFA> 2 % tend to form soap when alkali catalysts are used. Under optimised conditions (6:1 methanol-to-oil, NaOH), ASO biodiesel (B100) was produced in 92 % yield- higher than 78 % reported for biodiesel from another system but lower than 98 % from Vitex doniana seed oil, confirming ASO as a suitable feedstock [7][24].

GC-MS analysis in Table 2 showed that oleic acid is the dominant fatty acid (66.07 %), in close agreement with the 70.54 % reported for ASO and similar to the dominance of oleic acid in papaya seed oil [17]. The high monounsaturated content supports favourable combustion behaviour and acceptable oxidative and cold-flow properties [25]. The use of B100 is limited by viscosity, stability, and compatibility issues, which can be mitigated by blending with petroleum diesel [26][27] and the properties of B100, B10 (10:90 v/v), and B20 (20:80 v/v) are summarized in Table 3. The specific gravity of ASO B100, B10 and B20 (0.90, 0.85, 0.88) are within ASTM D6751 and EN 14214 limits, and values below 1 indicating a relative light fuel that may promote more complete combustion. Kinematic viscosity is one of the major properties of biodiesel which shows the flowability tendency of the fuel. It measures the fluidity and atomization properties of the fuel [28][29]. Kinematic viscosity at 40 °C was 7.70 mm²/s (B100), 4.80 mm²/s (B10), and 5.90 mm²/s (B20), with B10 and B20 within the ASTM D6751 range of (1-6 mm²/s) while B100 is slightly above. It was reported that elevated viscosity can impair injection and pumping, especially at low temperatures [30].

Moisture contents (0.30 %, 0.10 %, 0.22 %) exceed the 0.05 % maximum specified in ASTM D6751 and EN 14214, which may promote incomplete combustion and engine-performance degradation. The pH values (6.90,6.20. 6.70) are close to neutral, indicating limited corrosive risk under typical storage conditions. The flash point denotes the least temperature at which fuel vaporizes enough to get ignited, whereas fire point is the lowest temperature at which a fuel ignite fire for at least 5 seconds [31][32].  Flash points of 120 °C (B100), 75°C (B10), and 90 °C (B20), with corresponding fire points of 128°C, 95 °C and 105°C, are within ASTM D6751 and EN 14214 limits and are comparable to previously reported ASO biodiesel values around 110-128 °C, indicating reduced flammability hazard and improved handling safety [7].

Cloud points values (2 °C, 1 °C, -1°C) and pour points values (-1 °C, -5 °C, -8 °C) are within standard limits indicating suitable cold-flow behaviour for temperature- climate use. Cetane value is a vital characteristic of a diesel. It is used to evaluate the ignition quality of a diesel. Diesel with lower cetane value has a longer ignition delay when compared to diesels with high cetane value. The ignition delay time is the period that fuel is injected and starts to combust [33]. The cetane numbers [51,52, 50] obtained exceeded the minimum recommendation of 40, confirming good ignition quality and reduced ignition delay [33][34][17].  Iodine value evaluates the level of unsaturation of fatty acids in a biodiesel. It equally assesses the quality of biodiesel and the oxidative stability of biodiesel [35]. The iodine values of (55.00, 40.20, 47.30 g/100 g) are within the EN 14214 maximum of 120 g/100 g and are comparable with the 53.80 g/100 g reported for papaya seed biodiesel and contrast with the lower 16.39 g/100 g reported for ASO biodiesel in another study. Moderate iodine values indicated a balance between unsaturation and oxidative stability linked to high oleic content [25][18][36]. The acid values of (1.61, 0.96,1.02 mg KOH/g) and corresponding FFA contents of (0.81 %, 0.48 %, 0.52 %) exceeded the 0.5 mg KOH/g limit of ASTM D6751 and EN 14214, implying a potential risk of engine-part corrosion and suggesting the need for further purification or additives. Saponification value is used to check the level of impurities in a biodiesel, and also to determine the molecular weight of fatty acids methyl ester in the biodiesel [37].  The result showed saponification values of 160,125, 146 mg KOH/g) which met EN 14214 and indicated relatively high-molecular-weight methyl esters, which generally enhance stability, in contrast to higher 190 mg KOH/g reported for Beniseed oil biodiesel [18][38]. Peroxide value is a property of diesel indicating its oxidative level and tendency of the oil to become rancid.  Peroxide values of 7.40,4.80 and 5.66 mol/kg for B100, B10, and B20 are below the 10 mol/kg threshold often used to classify oils as stable, indicating that ASO biodiesel and its blends are relatively oxidation- resistant under the tested conditions. This aligns with the moderate iodine values and oxidative-stability data for avocado-rich oils [38][39]. Overall assessment showed that ASO is a viable waste-valorised feedstock for biodiesel; ASO B10 and B20 largely met international standards, with the main deviations being slightly elevated viscosity and moisture contents. The results support the use of avocado seed-based fuels as sustainable substitutes for petroleum diesel, consistent with circular-economy and decarbonisation goals.

Conclusion

This study successfully demonstrated the viability of avocado seed oil (ASO) – a non- consumable, abundant agro-waste material as an eco-friendly feedstock for biodiesel production through optimised transesterification with petroleum diesel blending. The seeds yielded appreciable oil content (13.50 %), underscoring its untapped potential amid global demands for second-generation biofuels. While the pure ASO biodiesel (B100) exhibited properties largely compliant with ASTM D6751 and EN14214 standards, minor exceedances in moisture content, acid value, viscosity and specific gravity highlight opportunities for refinement via advanced purification or pre-treatment techniques. Notably, the B10 and B20 blends met all stipulated limits, affirming their drop-in compatibility with existing diesel infrastructure and positioning ASO biodiesel as a viable petroleum diesel alternative. These findings contribute to circular economy principles by valorizing agricultural waste, potentially reducing reliance on food-competitive feedstocks and mitigating environmental pollution from avocado residues. Future research should explore long-term engine performance, oxidative stability enhancements, and life-cycle assessments to scale ASO biodiesel for commercial deployment in regions like sub-Saharan African where there is high production of avocado.

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