Introduction

The use of oil extracted from the seeds for the synthesis of biodiesel has been widely reported [1,2,3,4], but there is a limited literature available on the use of essential oil obtained from leaves for the audience to study. Also, the use of acids such as HCl, H2SO4, or organic acid to reduce the high-free-fatty acid (FFA) oils for easy biodiesel production has been reported [5, 6]. Literature on the use of bio-adsorbents such as coconut husk bagasse has also been reportedly used [7], but no single report has ever specified varieties such as dwarf green coconut husk ash (DGCHA) as bio-adsorbents for the reduction of high FFA oils. Furthermore, banana peels have been reportedly used as a base catalyst for the synthesis of biofuel [8], but no reports have been found on the specific varieties used for the developed catalysts. Lastly, the cost estimates of biodiesel production from these materials have never been reported anywhere. Therefore, this study employed the stem distillation method to extract oil from Croton heliotropiifolius Kunth leaf oil (CHKLO). The physicochemical properties of the oil were determined using the AOAC, 1997, and Wij’s methods. The high acid value of the oil was reduced using DGCHA. A new novel catalyst developed from the calcined Karpuravalli banana peel powder (CKBPP) variety was characterized using the FTIR, XRD-EDX, BET, TGA, ZETA, BET, and XRF-FS analyzers. Biodiesel production was carried out using microwave-assisted equipment, and the produced diesel was characterized and compared with the biodiesel recommended standard. The strength of the catalyst was determined via a catalyst reusability test, while the effectiveness and acceptance of biodiesel were estimated via cost analysis.

Croton heliotropiifolius Kunth is a plant native to Brazil (Northeast, Southeast, West-Central), Mexico (Central, Gulf, Northeast, Southeast, Mexico), Panamá, Paraguay, Peru, and Honduras. In Africa, it has been reportedly found in northern Nigeria. However, the leaves have been reportedly rich in oil, which can be used as a major material for biodiesel synthesis [9].

Bagasse can be obtained from Coconut husk that can be used as a bio-adsorbent for acid value reduction. Its application as a bio-adsorbent for acid reduction will not only add value to its existence but also solve the problem of waste disposal [7]. The economic advantages of biomass valorization includes but not limited to its availability, cost effective, and sustainable [10, 11].

Banana peel is a banana waste that has been reported to be rich in potassium. However, the potassium contents of calcined Karpuravalli banana peel powder (CKBPP) varieties have been reportedly high. Owing to the peel, which accounts for 30–40% of the weight of the banana, generates the majority of the garbage created by eating bananas, amounting to over 3.5 million tons of waste each year. This peel poses disposal problem, hence it can be used as a biobase for biodiesel synthesis.

The design of experiments using statistical tools such as RSM, ANN, ANFIS, and others has been found in the literature to determine the optimum biodiesel yield, but the central composite rotatable design (CCRD) was reported to be the most effective design for biodiesel synthesis [1, 2].

Therefore, this research employed the steam distillation process to obtain the oil from the green leaves of Croton heliotropiifolius Kunth. The high acid value oil was converted to low acid value oil using coconut husk bagasse as a bio-adsorbent. The reduced acid value oil was converted to biodiesel using a new novel heterogeneous catalyst developed from calcined Karpuravalli banana peel powder (CKBPP). Process optimization was carried out using central composite rotatable design, an allied of response surface methodology. The derived catalyst was tested for its reusability, and the properties of the biodiesel were compared with recommended standard. For fully acceptability of biodiesel to replace conventional diesel, cost estimate of biodiesel production was evaluated.

Materials and methods

Materials

Croton heliotropiifolius Kunth leaves (CHKL) were harvested freshly from a farm located in Kano State, Nigeria (longitude: 5.324928° N, latitude: 6.131293° E). The leaves were washed with distilled water to remove dirt and dried at room temperature for two days.

5 kg of dwarf green coconut husk (DGCH) was collected from Ozoro market, Delta State, Nigeria. The DGCH, also known as bagasse, was made into smaller sizes for further processing. Karpuravalli banana peels (5 kg) were obtained from a farmer in Elu community, Delta State, Nigeria.

Methods

Steam distillation

500 g of the CHKL plant was added to the stainless steel extractor, and the steam at 120 oC was added through an inlet to release the plant’s aromatic molecules and turn it into vapor. The vaporized CHKL compounds are then released into a condenser containing two separate pipes (a hot water outlet and a cold water inlet), and the vapor is then cooled back to form liquid oil. The liquid oil (liquid by-product) was collected from the condenser and kept in a separating funnel; two layers were noticed: the oil top layer and the water bottom layer. The oil was removed from the water through gravity settling and dried by heating at 100 oC to obtain pure essential oil, CHKLO. The CHKLO was examined for its properties as bio-oil using AOAC, 1997.

Preparation of activated carbon-based bio-adsorbent from coconut husk bagasse (CHB)

CHB was made into smaller particle sizes of 25 mm, cleaned, and heated in a local oven to form ash carbon. The ash carbon formed was activated with 0.1 N KMnO4 for a complete 48 h. The activated ash-carbon was cleaned with Aqua Dest, dried at 111 °C for 60 min, and then filtered with 150, 175, and 200 μm mesh, respectively.

Bio-adsorption of coconut husk bagasse activated carbon (CHBAC)

40 g of activated carbon was absorbed in 250 g of CHKLO and allowed to stay for 50 min. The mixture was properly mixed using a magnetic stirrer from one day to three days (24 h, 48 h, and 72 h), and the mixtures were filtered with filter paper, and the acid values were determined.

Karpuravalli banana peel as a catalyst

Karpuravalli banana peels were oven dried at 80 oC until their moisture content (< 0.001). The dried peels were milled into fine particles of 0.30 μm and calcined at 600 oC for 4 h in an electrical furnace. The calcined Karpuravalli banana peel powder (CKBPP) was allowed to stand for 48 h for proper cooling. Catalyst characterization was carried out so as to determine the morphological surface topography using SEM-EDX, and XRF-FT was employed to confirm the hetero-compound present in the CKBPP. The FTIR with model 3,116,465, made in Japan, was used to examine the functional groups and characteristic absorption bands. The ZETA potential analysis was carried out using Microtrac instruments that operate on the basis of dynamic light scattering (DLS) to determine the surface charge of the particles. The TGA (thermogravimetric analysis) was carried out to determine the thermal stability and the fraction of volatile components. BET-adsorption Langmuir and isothermal (QUANTACHROME, 1KE) and Hammett indicators were used to establish the pore space and sizes, pore diameter, and surface area.

Biodiesel synthesis from CHKLO

The low-acid CHKLO was produced in a 1000-mL reactor using Hisense 20-liter microwave-assisted equipment operated at 700 W with an existing inlet and outlet. The procedure was earlier reported by Ozioko et al. [11] with little modification. The microwave was modified by fitting a water-cooled reflux condenser and an external stirrer for the homogeneous reaction process. 200 mL of the oil was transferred to the flask, and a known amount of catalyst was dissolved in a measured volume of methanol and transferred to the oil in the reactor placed in the microwave. The heating power of the microwave apparatus was maintained at 120 W for the reaction to reach completion. At the end of the reaction, the high-speed centrifuge [MSLZL19; 16,000 rpm; ARC: 1.5 ml x 12; Max. RFC: 17000 (xg); timing range: 0–30 min] was used to separate the contents of the flask at 10,000 rpm for 5 min. The biodiesel was decanted, washed with distilled water, and then dried over CaCl2. The percentage yield of biodiesel was computed using Eq. (1):

$$\% {\rm{CHKLOB}} = \left({{{{\rm{WCHKLOB}}} \over {{\rm{WCHKLOU}}}}} \right) \times 100$$
(1)

Where: is the percentage of Croton heliotropiifolius Kunth leaves oil biodiesel, is the is the weight of Croton heliotropiifolius Kunth leaves oil biodiesel, and is the weight of Croton heliotropiifolius Kunth leaves oil used.

Design of an experiment for biodiesel synthesis

Experimental design for biodiesel synthesis was carried out using CCRD, an ally of RSM. Four constraint factors at four levels were considered, namely reaction time: T1 (60–90 min), reaction temperature: T2 (60–75 oC), catalyst concentration: T3 (3.0-4.5 wt%), and methanol-oil molar ratio: T4 (3–9 vol./vol.), respectively (Table 1). The design produced thirty experimental runs responsible for the biodiesel (Croton heliotropiifolius Kunth leaf oil biodiesel) experimental yield.

Table 1 CCRD experimental design
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Process optimization

Process optimization was carried out by determining the f-value, the p-value, the sum of the squares, the coefficient of determinations R2, R2 adjusted, R2 predicted, and adequate precision. The relationship between the chosen constraint and the response known as biodiesel is represented by the three-dimensional contour plots. The validated optimum yield was established by the average mean of three experimental data points.

Properties of biodiesel

The properties of the produced biodiesel were carried out using standard methods, and the results were compared with the recommended biodiesel standard such as the European and American standard values [12].

Reusability of CKBPP

The used catalyst was recycled after production and reused. The spent catalyst after recycling was refined by washing with ethanol, dried in an oven for 1 h at 150 oC, and reused in cycles. The reusability test was brought to an end at a point when the yield of biodiesel produced with recycled catalyst was greatly reduced. The data obtained was depicted using the Excel plots.

Results and discussions

Properties of extracted CHKLO

It was observed that the CHKL is rich in oil with an approximately 43.00%(wt./wt.). The properties of the CHKLO were determined using the AOAC, 1997 procedural methods. Displayed in Table 2 are the physical, the chemical, and the fuel properties compositions of the CHKLO for its suitability for biodiesel production. The viscosity which measures the oil’s resistance to flow, the value recorded in this study indicated the oil is heavy oil (heavy oil viscosity ranges from 12 to 100 cp.). The specific gravity which determines how dense the oil is shows that the produced oil specific gravity (0.82) was within the range recommended for crude oil. The acid value of the oil 5.32 mg KOH/g oil implies the oil is acidic and can only be used for biodiesel synthesis via acid reduction processes.

The saponification value which implies how much base is KOH needed is needed to saponify 1.0 g of CHKLO was obtained as 182.00 mg KOH/ g oil. The degree of saturation is indicated by iodine value. The value obtained in this study 78.02 g I2/100 g oil, shows the oil is highly unsaturated. The fuel properties such as cetane, HHV and smoke point, and flash point indicated the oil possesses fuel properties can be used as raw material for biodiesel production.

Table 2 Properties of the CHKLO
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Determination of acid value of the CHKLO

During the extraction process, as the temperatures increases, the triglyceride C = C (sp2) oil’s is broken and the oil’s acid value reached 5.32%. However, the oil’s acid value decreases as it adsorbs into CHBAC. Depicts in Fig. 1 are the plots with pore diameters of different ash sizes 170, 190, or 210 microns, the oil was immersed in bio-adsorbents for one to three days, and the acid values were determined. The results showed that the acid values varied from 4.52 to 3.50 (mg KOH/ g oil) after one day of immersion, the acid values reduced from 3.40 to 2.44 (mg KOH/ g oil) in the following day (two days), while the reduction in acid values from 2.00 to 1.40 (mg KOH/ g oil) were observed in the third day. These implied that the after immersion in bio-adsorbent, the smallest pore size (200 microns) had the lowest acid value (third day). This proved that CHBAC worked well as a bio-adsorbent to reduce the acid value of CHKLO. The presence of high lignin, cellulose, organic compounds, and hemicelluloses, lignin, and proteins contents of the CHB led to the selection of this solid biowaste material which contains macromolecules. These macromolecules contain adsorptive sites like double bond, amine, and hydroxyl groups, which can be used to adsorb acid value through ion exchange phenomena. Moreover, its ability to be recycled and reused as waste was an additional reason to be use bio-adsorbent.

Fig. 1
figure 1

Bio-adsorption of bagasse for acid value reduction

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Characterization of CKBPP

FTIR analysis

Data values obtained on the novel catalyst developed from calcined Karpuravalli banana peel powder (CKBPP) based on the FTIR were as depicted in Fig. 2. The different angles at a maximum decree of 2θ that are referred to as absorbance transmittance match the different vibrations of CKBPP atoms when exposed to the infrared portion of the electromagnetic spectrum. In this instance, the wave number on the figure’s IR spectrums is drawn from low 1035.2 IR to higher 3339.7 cm-1 IR bands are located in various print frequencies. The peaks found around at 1036.2 cm− 1 indicated the presence the primary, secondary, and tertiary alcohol, C–O stretch. The band at 1033.3 cm− 1 shows the primary or secondary, OH in-plane bend. The band at 1155.3 cm− 1 are classified as phenol or tertiary alcohol, OH bend, the peroxide, and the C–O–O-stretch. The peak region associated with 1200.2 cm− 1 shows the presence of the Epoxy, oxirane rings, Aromatic ethers, and aryl-O stretch. The value of the peaks found at 1315.6 cm− 1 depicts the presence of the Alkyl-substituted ether, and Cyclic ethers with large rings, and the C–O stretch. The band observed at 1371.7 cm− 1 provides the primary, secondary, and tertiary, both amine and aromatic CN stretch. The band displayed around 1442.5 cm− 1 reflect the carboxylate salt, the P–O–C, aromatic and aliphatic phosphates, the carbonate ion, sulphate, nitrate, phosphate, potassium, aluminate, silicate e.t.c can be found. Further band located at 1536.3 cm− 1 indicated the presence of the primary and secondary amine > N–H bend, Aromatic ring (aryl) such as C = C–C aromatic ring stretch, the olefinic (alkene) such as Alkenyl C = C tretch, aryl substituted C = C, and conjugated C = C. The band at 2922.2 cm− 1 depicts the presence of Saturated aliphatic (alkene/alkyl) such as methyl C–H asym./asym stretch, methylene C–H asym./sym stretch, methyne C–H stretch, methoxy, methyl ether O–CH3, C–H stretch, methylamino, N–CH3, C–H stretch, and the Acetylenic (alkyne) such as alkyne C–H stretch. The band at 3339.7 cm− 1 indicate the alcohol and hydroxyl compound such as hydroxyl group, H-bonded OH stretch, normal polymeric OH stretch, Dimeric OH stretch, internally bonded and non -bonded hydroxyl group, OH stretch, primary, secondary, tertiary alcohol, OH stretch, phenols, OH stretch, the ether and oxy compound of methoxy, C–H stretch (CH3–O-), the primary and secondary amino of aliphatic and aromatic primary amine NHH-stretch, aliphatic and aromatic secondary amine > N–H stretch, heterocyclic amine > N–H stretch, and imino compounds = N–H stretch, the thiols of S–H stretch, and the common inorganic ions such as ammonium ion.

Carbonate salts undergo thermal decomposition, yielding K-Na-Mn-P oxide and trioxocarbonate as the major minerals in the CKBPP catalyst. These attributes contribute to the choice of Karpuravalli banana peels as novel heterogeneous catalyst in this study.

Fig. 2
figure 2

FTIR analysis of the CKBPP

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TGA analysis

The thermogravimetric analysis (TGA) analysis data obtained as a thermal decomposition of the CKBPP which resulted in weight loss are presented in Fig. 3. Using PTA (PerkinElmer thermal analysis), the composition kinetic analysis of thermal breakdown was as displayed. The loss in weight of the catalyst was found to significant as the temperature increased above 500 oC, there was greater reduction in concentration of the CKBPP indicating the conversion of the CKBPP to bio-ash for the catalytic activities.

Fig. 3
figure 3

TGA analysis of the CKBPP

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XRD-FS analysis

The XRD-FS analysis data of the CKBPP are presented in Table 3, indicating the presence of bio-base in the novel catalyst used for the conversion of oil to biodiesel. The presence of this salts supported the FTIR analysis findings of finger prints found around the peaks found in 2922.2 to 3339.7 cm− 1 depicts the presence of single bonds of hydrogen (C – H) found in the CKBPP. During biodiesel synthesis, these compounds have the ability to absorb the excess water found in the CHKLO and are always stable (non-expanding and charge imbalanced).

Table 3 XRD-FS analysis data on CKBPP
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SEM analysis

The SEM analysis morphological results at magnification of 1500X are as presented in Fig. 4. The image indicated whitish-rigid-coherent formation with cracked structures indicating the thermal effects on the catalyst. The whitish nature could be attributed to the presence of calcium and potassium salts. The porous look displayed indicated the catalyst’s absorbent character permits the adsorption of alcoholic impregnation on its surface during the transesterification reaction.

Fig. 4
figure 4

SEM analyses of CKBPP at magnification 1500X

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BET adsorption isotherm analysis

Displayed in Table 4 are the data on the BET adsorption isotherm of the CKBPP showing the ascending increased in pore width, cumulative pore volume, and the cumulative surface area.

Table 4 DFT method, pore size distribution
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A plot of relative pressure was carried out against the quantity absorbed in Fig. 5. It was observed that the isotherm is characteristic of non porous or micro porous absorbent. The knee of the isotherm is an indication of completion of monolayer coverage and initiation of multilayer adsorption. Also, the adsorption and desorption branch coincides, and this is known as type II adsorption isotherm found in pyrolized empty pal bunch. BJH also known as Barrett-Joyner-Halenda, method was used to study pore size distribution of porous materials CKBPP as presented in Table 4 indicated the values of increase in porous volume against the pore width. Pores with pore width above 60 nm are not always considered for adsorption and desorption isotherm of BET analysis of the CKBPP. A large pore diameter and small surface area indicated a low adsorption capacity, while a small pore diameter, will limit the diffusion of adsorbates and gases, which enhances the shielding effect for molecules with larger diameters. However, the average width < 2 nm is referred to as micro-porous, between 2 nm and 50 nm can classified as mesoporous, and > 50 nm can be said to be macro-porous. In this case, the lowest was 10.32 nm while the highest was 58.52 nm for the pyrolized PB3, which can be classified as both mesoporous and macro-porous isotherm. This makes the sample material as a good candidate for gas adsorption.

Fig. 5
figure 5

Isotherm plot

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ZETA data analysis

The ZETA data analysis of the CKBPP characterization is as displayed in Fig. 6. The images illustrate the correlation function and coefficient, along with the time variant. In a suspension in which zeta potential was measured, a positive zeta potential indicates that the dispersed particles have a positive charge with excellent stability, whereas a negative zeta potential indicates that the dispersed particles have a negative charge and are thought to be agglomerated. As a result of the high positive charge found in the CKBPP value (Zeta average = 74.60 d.nm, intensity of 76.2% for the first peak) in this study, the catalyst activity was stable during the transesterification reaction for biodiesel synthesis.

Fig. 6
figure 6

ZETA analysis of CKBPP characterization

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Production of biodiesel and its process optimization

Presented in Table 5 are the results on the variable combinations, the experimental yield (ECHKLOB), and the predicted yields (PCHKLOB) by the RSM infusing CCRD. The lowest biodiesel yield was observed at experimental runs 11 with 91.49%(wt./wt.) biodiesel yield, while the maximum experimental yield was obtained at 95.60%(wt./wt.), runs 21. However, all the biodiesel experimental yields were above 90%, indicating that the experimental findings were successive and the variables constraints have significant effects on the yield.

Table 5 Variable combinations, experimental, and the predicted data
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Process optimization was carried out in three steps: the first step was determination of analysis of variance which indicated the level of significant, the f-values, the p-values, the means square, the degree of freedom, the sum of square, and the source. The results obtained are presented in Table 6. These results indicated that all variables considered were found to have mutual significant on the biodiesel yield. The second step is via fit statistic and the model equation. The fits statistic indicated a high coefficient of determinations for R2, adjusted R2, and predicted R2, respectively. This implies that the polynomial model quadratic is suitable for adequate representation of biodiesel yield (Eq. 2).

Table 6 2nd order of polynomial and fits statistic
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$$\eqalign{{\rm{CHKLOB\% }}(wt./wt.) & = 93.17 - 0.065{K_1} - 0.3817{K_2} \cr & + 0.19{K_3} + 0.4033{K_4} \cr & + 0.3488{K_1}{K_2} + 0.42{K_1}{K_3} \cr & + 0.2613{K_1}{K_4} - 0.4212{K_2}{K_3} + 0.435{K_2}{K_4} \cr & + 0.6813{K_3}{K_4} - 0.1312K_1^2 \cr & + 0.36K_2^2 - 0.3337K_3^2 + 0.31K_4^2 \cr}$$
(2)

The desirability-three-dimensional contour plots of the duo variables on the predicted response (PCHKLOB) while keeping the other variable constant are the third step of model and validation of process optimization. These plots are presented in Fig. 7, which shows the mutual interaction among the variables.

Fig. 7
figure 7

Desirability and the three dimensional contour plots

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Meanwhile, the statistical tools predicted an optimum biodiesel yield of 95.285% (wt./wt.) at T1 = 90 min, T2 = 60 oC, T3 = 4.5% (wt.), and T4 = 9 (vol./vol.), respectively. This value was validated in triplicate, and the average mean value of 95.10% (wt./wt.) was established as the optimal biodiesel yield. This confirmed that the RSM has proved to be a statistical optimization tool for the conversion of CHKLO to CHKLOB.

Properties of CHKLOB

The qualities of biodiesel produced were examined via the properties determination and the results were compared with recommended biodiesel standards as presented in Table 7. The results of findings indicated that produced CHKLOB conformed with the recommended standard [12], hence, the fuel can replace conventional diesel by blending or direct usage.

Table 7 Properties of CHKLOB
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Ctalyst reusability test

Shown in Fig. 8 are the results on the catalyst reusability test indicating the strength of CKBPP. It was observed that the biodiesel yields were uniformly constant for the first four cycles, a little decrease in the yield at fifth cycles, but a significant drop in the yield was noticed at sixth cycle as a results of catalytic activities been hampered by leaching via pore spaces. The basic strength was weak and hence, the recyclability test was altered at fifth cycles. This indicated that CKBPP is a good heterogeneous catalyst that can be employed in industries.

Fig. 8
figure 8

Reusability test data plot

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Cost implication of CHKLOB

The produced biodiesel was examined for its cost effectiveness and reality by evaluating the cost of production. Variables such as the cost of raw materials, the cost of purification, the cost calcinations, the cost transportation, and other factors contribute to the cost of biodiesel production. These factors were considered appropriately. The results indicated that the cost of producing 20 L of CHKLOB is presented in Table 8. These results show that the cost of producing 1 L of biodiesel is (7100/20) = 350.00/L. The cost of 1 L of world diesel as of June 12, 2024 is 1,100/L. Estimating this in dollars ($), the cost of producing 20 L of biodiesel (CHKLOB) was $4.73 at 1,500 to $1.

Table 8 Cost estimate of CHKLOB
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Limitations

The article is limited to laboratory scale work. A design expert trial version was used for the experimental design.