Adewale Adewuyi ,Adole I.Ogagbolo ,Woei Jye Lau ,Rotimi A.Oderinde

1 Department of Chemical Sciences,Faculty of Natural Sciences,Redeemer’s University,Ede,Osun State,Nigeria

2 Department of Chemistry,Faculty of Natural Science,University of Ibadan,Ibadan,Oyo State,Nigeria

3 School of Chemical and Energy Engineering,Universiti Teknologi Malaysia,81310 Skudai,Johor,Malaysia

Keywords:Adsorption Co-precipitation Deteriorated vegetable oil Free fatty acid (FFA)Langmuir

ABSTRACT Deterioration and loss of quality of vegetable oil is a big challenge in the food industry.This study investigated the synthesis of nickel ferrite(NiFe2O4)via co-precipitation method and its use for the removal of free fatty acids(FFAs)in deteriorated vegetable oil.NiFe2O4 was characterized using Fourier transformed infrared spectroscopy (FTIR),X-ray diffraction (XRD),thermogravimetric (TG) analysis,Brunauer-Emm ett-Teller (BET) surface area,transmission electron microscopy (TEM),scanning electron microscopy(SEM) and energy-dispersive X-ray spectroscopy (EDX).Synthesis of NiFe2O4 was confirmed by characterization,which revealed a BET surface area of 16.30 m2·g-1 and crystallite size of 29 nm.NiFe2O4 exhibited an adsorption capacity of 145.20 L·kg-1 towards FFAs with an 80.69% removal in a process,which obeys Langmuir isotherm and can be described by the pseudo-second-order kinetic model.The process has enthalpy (ΔH) of 11.251 kJ·mol-1 and entropy (ΔS) of 0.038 kJ·mol-1·K-1 with negative free energy change (ΔG),which suggests the process to be spontaneous and endothermic.The quantum chemical computation analysis via density functional theory further revealed the sorption mechanism of FFAs by NiFe2O4 occurred via donor-acceptor interaction,which may be described by the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO).The study showed NiFe2O4 to be a potential means that can remove FFAs from deteriorated vegetable oil.

1.Introduction

Oxidation of vegetable oil is an unwanted reaction,which majorly affects unsaturated fatty acids [1].It eventually leads to deterioration and loss of quality of vegetable oil.The reaction is of economic concern in food industry because it leads to the development of unwanted flavor and odor in vegetable oil or vegetable oil containing food products.The deterioration process is promoted by the presence of oxygen,metal,heat or light[2].The degradation becomes more apparent when vegetable oil is used for frying over time.Such degradation affects the quality of the vegetable oil most especially when it is exposed to air during thermal heating or frying [3].

Several factors are known to influence the deterioration of vegetable oil.These include temperature,accessibility to oxygen,composition of vegetable oil,frying operations and surface to volume ratio [4].Vegetable oils are triglycerides and when they undergo deterioration,the process leads to hydrolysis of triglycerides with the formation of free fatty acids (FFAs),off-flavor and odour [4].Report by Karakaya and S?ims?ek [5] reveals the presence of polar compounds in degradation of soybean,corn,olive and hazel nut oils.Previous study has shown oil frying and oil-water has significant increase in acid value due to moisture [6].Study by Nawaret al.[7] showed that hydrolysis of triglyceride yields mono and di glycerides as well as FFAs.FFAs are known as one of the major products of degradation of vegetable oil,which must be removed.

Several methods have been documented for the removal of FFAs in deteriorated vegetable oils,which helps to prolong shelf life and reduce their susceptibility to oxidation[8,9].One of these methods is alkali deacidification.During the refining of crude vegetable oil,alkali deacidification involves the neutralization of FFAs by precipitating it out as soapstock(byproduct)while the neutral oil is separated mechanically.The soapstock formed poses threat to the quality and quantity of neutral oil separated [8].The challenge from the soapstock is the major challenge of the alkali deacidification method,which limits its use in refining of deteriorated vegetable oils.

Other methods reported for FFAs removal are physical deacidification,enzymatic deacidification [8] and solvent extraction [10].However,physical deacidification had always exhibited better advantages over chemical and alkali deacidification[8].Adsorption has been shown as an effective method for the removal of FFAs from deteriorted vegetable oils due to its efficiency,affordability and environmental friendliness [11].Several materials have been developed in the past as adsorbent for adsorption process but most of them have shown drawbacks,which discourages their use.However nanomaterials have shown prospects as good adsorbent for adsorption process.This study proposes the use of metal nanoparticle for the removal of FFAs from deteriorated vegetable oil.

Nanomaterials are materials in the dimension 1-100 nm with unique properties such as surface area,size,porosity,optical,electrical,strength and magnetic.These give them advantage over other materials [12,13].They have high surface area to volume ratio,which has given them application as adsorbents.Nanomaterials may be organic or inorganic;those containing metals are of particular interest due to the unique properties they exhibit such as their outstanding catalytic property.Iron oxide nanomaterial is known to have large surface to volume ratio,compatibility,superparamagnetic actions and chemical stability [14].This study proposes the use of iron oxide nanomaterial due to the high surface area it exhibits;however this property is hampered by the chemical reactivity of iron oxide.It is easily oxidized in air forming aggregates that reduces the surface area and magnetism,thus reducing its capacity as adsorbent.Therefore,it is important to improve on the properties of iron oxide by protecting and enhancing its stability towards oxidation.Although different approaches have been used in the past [15-18] but still have some shortcomings.To address this,this work proposes the synthesis of spinel ferrite(NiFe2O4) by coprecipitation method usingAdansonia digitataoil as a capping agent.The aim of the work is to synthesise NiFe2O4and evaluate its use as an adsorbent for the removal of FFAs from deteriorated vegetable oil.

2.Materials and Methods

2.1.Materials

Iron (III) chloride hexahydrate (FeCl3·6H2O),nickel (II) chloride hexahydrate (NiCl2·6H2O),sodium carbonate (Na2CO3),sodium hydroxide (NaOH),hydrochloric acid (HCl),n-hexane,anhydrous sodium sulphate (Na2SO4) and all other chemicals used in this study were purchased from Aldrich Chemical Co.,England.Melon oil (Citrullus colocynthis) used in this study was purchased from a market in Osogbo,Nigeria.Adansonia digitataseed oil was used as capping agent.The fatty acid composition ofAdansonia digitataseed oil has been reported by Adewuyiet al.[19]to contain mainly C18:1 (36.55%) and C18:2 (28.19%) fatty acids.The melon oil used in this study was continuously used for deep-frying until it became deteriorated with FFA value reaching 90 g·kg-1to become deteriorated melon oil(DMO).The FFA value was determined by titrating the oil with 0.1 mol·L-1NaOH using phenolphthalein as indicator.

2.2.Synthesis of NiFe2O4 particles

NiFe2O4particles were synthesized using FeCl3·6H2O and NiCl2-·6H2O salts.Aqueous solutions of NiCl2·6H2O (0.2 mol·L-1) and FeCl3.6H2O (0.4 mol·L-1) were mixed and stirred continuously for 1 h.Adansonia digitataoil (10 ml) was added in order to control particle growth and as a capping agent.Solution of NaOH(2 mol·L-1) was added dropwise until precipitate appeared and emulsion turned black within the pH 10-12.The color change indicated the formation of NiFe2O4.It was heated to 80°C,stirred constantly for 2 h,cooled to room temperature.The residue was filtered,washed severally with water and ethanol.The residue was dried overnight in the oven at 105 °C before transferring to the furnace at 550 °C for 18 h.

2.3.Determination of fatty acid composition of melon oil

Fatty acid composition of melon oil was detemined as previously described elsewhere [20].To achieve this,melon oil was transesterified using 2% sulphuric acid in methanol.The product was a mixture of methyl esters,which was extracted with ethylacetate,washed with distilled water until free of acid and dried over Na2SO4.The mixture was concentrated using a rotary evaporator and the fatty acids were determined as methyl esters using gas chromatography (GC).Agilent 6850 series GC equipped with an FID detector and a DB-225 capillary column was used.The column temperature was kept at 160 °C for 2 min,which was gradually increased to 230°C at 4°C·min-1for 10 min.Nitrogen was the carrier gas at a flow rate of 1.5 ml·min-1.The detector and injector temperatures were maintained at 250 °C and 230 °C,respectively at a split ratio of 50:1.

2.4.Characterization of NiFe2O4 particles

The functional groups in NiFe2O4were determined using FTIR(Perkin Elmer,spectrum RXI 83303,MA,USA) with values were recorded in the range of 400-4500 cm-1.Brunauer-Emmett-Teller(BET) surface area was determined by N2gas adsorption(Micromeritics,TriStar II 3020 version 3.02).TG analysis was carried out using TGA Q500 V20.The X-ray diffraction pattern of NiFe2O4was measured using X-ray diffractometer (Smartlab,Rigaku) at a scanning speed,range and step width of 8.255 (°)·min-1,3.000°-100.000° (2θ) and 0.020°,respectively with filtered Cu Kβ radiation operated at 40 kV and 30 mA.Surface morphology of NiFe2O4was examined on TEM (HT7700,Hitachi).In order to confirm the elemental composition,SEM equipped with EDX detector (TM3000,Hitachi) was used.

2.5.Adsorption of FFA on NiFe2O4 particles

The sorption of FFAs by NiFe2O4particles was carried out by contacting 100 ml of DMO with 0.05 g of NiFe2O4particles in a 250 ml Erlenmeyer flask while stirring continuously for 60 min at 120 r·min-1.DMO was intermittently withdrawn as the stirring continued to analyze for FFA content by titrating the withdrawn sample with 0.1 mol·L-1NaOH using phenolphthalein as indicator.An initial concentration range of 50-90 g·kg-1FFA content was used.The sorption process was repeated varying the weight of NiFe2O4particles in the range of 0.05-0.1 g and at different temperatures (303-323 K).The adsorption capacity of NiFe2O4,qe(L·kg-1) was calculated using expression:

whereCoandCeare the initial and final amounts (g·kg-1) of FFAs,respectively,Mrepresents the mass (g) of NiFe2O4andVstands for the volume (L) of DMO.

2.6.Desorption studies

Removal of FFAs from the surface of NiFe2O4particles was studied using the following solvents;hexane,diethylether,chloroform,water,methanol,ethylacetate,and chloroform:methanol(1:2).To achieve this,0.05 g of NiFe2O4particles was contacted with DMO having initial concentration of 90 g·kg-1FFA in 100 ml Erlenmeyer’s flasks at room temperature while stirring for 60 min at 120 r·min-1.The FFAs loaded NiFe2O4particles was dried at room temperature for 48 h.The FFAs loaded on NiFe2O4particles were removed in these solvents to estimate its desorption capacity.The solvents were separately poured into different 250 ml Erlenmeyer’s flasks containing 100 ml of the solvent.These were stirred continuously at room temperature for 60 min at 120 r·min-1.The amount of FFA desorbed was determined by titrating with 0.1 mol·L-1NaOH using phenolphthalein as indicator.Desorption of FFA from NiFe2O4was calculated as:

whereqaandqdare total amount of FFA adsorbed at the surface of NiFe2O4at equilibrium and the total amount of FFA desorbed from the surface of NiFe2O4.

2.7.Quantum chemical parameters

Sorption of FFAs by NiFe2O4was subjected to theoretical calculations using density functional theory (DFT) electronic structure programs at B3LYP/6-31G level theory using Spartan 14.1 software.Molecular electronic structure of FFA was modeled,which covers the distribution of frontier molecular orbitals in order to establish the interaction of FFA with NiFe2O4.The absolute hardness(η)was calculated as [19]:

3.Results and Discussion

3.1.Synthesis of NiFe2O4 particles

The bonding behavior of the synthesized NiFe2O4particle was examined using FTIR and the result is presented in Fig.1a.

The spectrum revealed peak at 3442 cm-1,which was assigned to the vibrational frequency of O—H group on its surface.The OH group may have been attached to the Fe ions;perhaps the peak may also account for the moisture on the nanoparticles of NiFe2O4[21,22].The peaks at 1654 and 1486 cm-1were assigned to the bending vibration of H2O molecules and OH groups attached to Fe ions [23].The peak observed at 1820 cm-1was attributed to the vibrational frequency of anions(Cl-),which may not have been completely removed during washing [24].The vibration at 1385 cm-1was ascribed to the frequency of carbonate group due to the presence of CO2at the surface of the particles[25].The peaks at 585 and 385 cm-1corresponds to the stretching frequencies due to tetrahedral Fe—O and octahedral Ni—O [26,27].

Fig.1. FTIR (a),XRD (b),TG analysis (c) and BET (d) of NiFe2O4.

The diffractogram is shown in Fig.1b,which revealed a singlephase spinel structure.The most intense peak is at 2θ=35.60°with a plane spacing corresponding to(3 1 1),the plane of NiFe2O4particles [26,28].Subsequent planes corresponding to (2 2 0),(2 2 2),(4 0 0),(4 2 2),(5 1 1),(4 4 0),(6 2 0),(5 3 3) and (4 4 4) were observed at 30.30°,37.30°,43.32°,52.82°,57.38°,62.95°,66.40°,78.20° and 81.20°,respectively [17,29,30].The crystalline NiFe2O4may be indexed as inverse spinel and face centered cubic structure[31].The crystallite size was determined using X-ray line broadening from the reflections of (3 1 1) and Debye-Scherrer’s formula[32].

From the formula,Dis the average crystallite size of NiFe2O4,Kis a constant taken as 0.89.λ is the X-ray wavelength (0.15406 nm),β represents the full width of diffraction line at half of the maximum intensity (FWHM),θ is the Bragg’s angle (in radian) where peak(3 1 1) is observed [17].The crystallite size is 29 nm.

Fig.1c shows the TG result of the self-synthesized NiFe2O4.The data showed an initial loss in mass in the range of 50-155 °C,which might be associated with the loss in water molecules and any other volatile molecule that may have adsorbed on the surface of NiFe2O4.Similar observations was reported by Sivakumaret al.[26],which indicated spontaneous combustion of the particles.The loss within this range corroborate the vibration observed at 1654 cm-1in the FTIR spectrum,which was associated to the bending vibration of H2O molecules.The loss in mass from 155 to 810 °C might be due to the dehydration of OH group in the spinel structure,which involves inter and intramolecular transfer reactions,and formation of metal oxides and the spinel phase[26,33].A sharp loss in mass was observed at temperature above 810 °C,which might further be due to the spinel phase as well as the appearance of cryatallization.

The surface area was measured using BET equation [34] from the N2adsorption-desorption isotherm of NiFe2O4as presented in Fig.1d.The BET surface area is 16.30 m2·g-1as calculated by linear part of the BET plot.The total pore volume (atP/P0=0.900) is 0.048 cm3·g-1and the adsorption average pore diameter is 1.0455 nm.The BET isotherm is of type II,which is typical of mesoporous adsorbents[35].The SEM surface mapping and EDS results are shown in Fig.2a and b,respectively.The surface mapping and EDS confirmed the elemental composition of NiFe2O4with the presence of Ni,Fe,C and O.The morphology of NiFe2O4was further studied by TEM.The TEM images shown in Fig.2c and d,reveal the presence of particles with different shapes and dimension.These images show a clear lattice fringe of nanoparticles,which might be considered an oriented attachment,like a sheet-like nanostructure.The different shape and dimension of the particles is a reflection of the crystallite dimension and ionic radii.This suggests the coexistence of both monocrystalline and polycrystalline particles in NiFe2O4.

3.2.Adsorption process

Fig.2. Surface mapping (a),EDS (b),TEM image at 100 nm (c) and TEM image at 20 nm of NiFe2O4.

Fig.3. (a)Percentage removal of FFA,(b)adsorption capacity of NiFe2O4 towards FFAs and(c)Effect of NiFe2O4 mass on percentage removal of FFAs and adsorption capacity.

Table 1 Fatty acid composition of melon before and after treatment with NiFe2O4

Melon oil is a household vegetable oil in Africa,especially in Nigeria where it is used for cooking and frying.It is very important to study how derioriated form of this oil (DMO) can be treated in order to be reused and avoid being discarded in the environment as an environmental waste.The sorption of FFAs by NiFe2O4was achieved by periodical analysis of DMO during treatment.Equilibrium was attained after 40 min of treatment.The plot of percentage removal of FFAs by NiFe2O4overtime is shown in Fig.3a.The maximum removal attained was 80.67%.The percentage removal increased with increase in the initial amount of FFA in DMO.It is also obvious that over 40%of the total FFAs removed was achieved in the first 5 min of treatment.This observation revealed the sorption process to have a fast initial uptake since almost 50% of the overall removal was achieved within 5 min.This observation increased with the initial concentration of FFAs.This observation might be due to the fact that more species of FFAs were available for interaction with NiFe2O4as the amount increased from 50 to 90 g·kg-1.The fast initial uptake might be due to the ease of accessability of the active site by the FFAs,the active sites for adsorption may have been readily available and easily accessed for interaction by FFAs within the first 5 min before accumulation of FFAs at the active sites occurred.

NiFe2O4exhibited an adsorption capacity of 145.20 L·kg-1towards FFAs.The adsorption capacity of NiFe2O4for FFAs at different concentration against time is presented in Fig.3b.The capacity of NiFe2O4towards FFAs increased overtime as the amount of FFAs increased from 50 to 90 g·kg-1.

The fatty acid composition of melon oil before and after treatment with NiFe2O4is presented in Table 1.The most abundant fatty acid in melon is C18:2 (55.88%) and the least being C14:0(0.07%).After treatment,the fatty acid composition was changed due to the removal of FFAs.The unsaturation decreased from 78.28%before treatment to 72.30%after treatment while the saturation increased from 21.72% before treatment to 27.70% after treatment.This is a strong indication that most of the removed FFAs were unsaturated fatty acids.

It is also obvious from the fatty acid results that NiFe2O4removed most of the short chain fatty acids in the DMO while the long chain fatty acids were intact.This might be due to the preference of NiFe2O4towards short chain fatty acids as result of their small molecular weights compared to the long chain fatty acids with high molecular weight or it may be that the long chain fatty acids were not hydrolyzed during deterioration of the melon oil and were not available for sorption during the treatment.The effect of adsorbent weight on percentage removal of FFAs as well as adsorption capacity is presented in Fig.3c.The percentage removal of FFAs increased as weight of NiFe2O4increased from 0.05 to 0.1 g.This might be due to an increase in effective surface area as the weight increased making more active sites available for the sorption process.However,the adsorption capacity decreased as weight of NiFe2O4increased,which may be due to a decreased mass transfer to active surface area ratio.

3.3.Kinetic studies

Data obtained for the sorption of FFAs by NiFe2O4was fitted for pseudo-first-order,pseudo-second-order,intra-particle diffusion,elovich and liquid film diffusion models.Linearized expression for pseudo-first-order is given as:

whereqeandqtare the amounts of FFAs at equilibrium and timet,k1represents the pseudo-first-order rate constant (min-1) and t is time (min).In (qe-qt) was plotted againsttfrom whichk1andqewere obtained from the intercept and slope of the plot.The plot gave anr2value of 0.923 as shown in Table 2.The data was subjected to pseudo-second-order as described in the following equation:

wherek2(L·kg-1·min-1) represents the adsorption rate constant.Values ofqe,k2andh(initial sorption rate) were calculated from the slope and intercept of the plot oft/qt versus t.Ther2value was 0.998 with aqevalue of 156.25 L·kg-1.The data were further fitted for intra-particle diffusion model to understand sorption rate limiting step.This was achieved as follows:

Table 2 Kinetic model parameters for the sorption of FFA on NiFe2O4

From Equation (7),C (L·kg-1) is the constant that express the thickness of the boundary layer.Kidrepresents the intra-particle diffusion rate constant (L·kg-1·min-1/2).Plottingqt versus t1/2gave a straight line withr2value of 0.929,which suggests the removal of FFAs to be controlled by intra-particle diffusion.Values ofCandKidwere obtained from the intercept and slope,of the plot,respectively.However,the regression of the plot did not pass through the origin,which suggests that intra-particle diffusion could not have been the sole rate-limiting step during the sorption of FFAs by NiFe2O4.

To further check for this,data were further subjected to liquid film diffusion model,by subjecting data to equation:F(qe/qt)represents the fractional attainment of equilibrium andKfdis the adsorption rate constant.The plot of-ln(1-F)againsttgave anr2value of 0.943,which further corroborate the fact that film diffusion must have played a role in the rate-limiting step.The data were further treated for Elovich model using equation:

β is the extent of surface coverage(L·kg-1)and α represents the initial adsorption rate (L·kg-1·min).α and β were obtained from the intercept and slope of the plot ofqtagainst Int,which gave anr2value of 0.967.

3.4.Isotherms

Data for the sorption of FFAs by NiFe2O4were subjected to three isotherm models,i.e.,Temkin,Langmuir and Freundlich.The Temkin model is expressed as:

B(J·mol-1)=RT/bwherebis the Temkin constant.T(K)is the absolute temperature,A(L·g-1) is the Temkin isotherm equilibrium binding constant andR(8.314 J·mol-1·K)is the gas constant.Graph ofqeagainst lnCegave a straight line whereAandBwere obtained from the slope and intercept.The results are shown in Table 3.The linear expression for Langmuir isotherm model is given as:

Table 3 FFA sorption parameters for Temkin,Langmuir and Freundlich models

From Eq.(12),Ce(g·kg-1) is the equilibrium amount of FFAs,qe(L·kg-1) represents the amount of FFAs removed at equilibrium,Qo(L·kg-1) is the maximum monolayer coverage capacity andKL(L·mg-1) represents the Langmuir isotherm constant.Plot ofCe/qeagainstCe,gave a straight line with slope being 1/Qoand 1/QoKLbeing the intercept.The essential feature of Langmuir isotherm is described asKL,which is expressed as:

KLrelates to the energy of sorption of FFAs whileCois the initial amount of FFA.Previous study [36] has shown that whenRL＞1 sorption process is considered to be unfavorable,whenRL=1,sorption process is considered to be linear,when 0 ＜RL＜1,the process is considered to be favored,and irreversible ifRL=0.However,the Langmuir isotherm was built on the assumption that the uptake of FFAs took place on homogenous surface by monolayer sorption.This also assumes a uniform energy of adsorption at the surface of NiFe2O4.

In this study,sorption of FFAs on NiFe2O4is Langmuir isotherm favored because the value obtained forRLis within 0 ＜RL＜1 as shown in Table 3.The sorption energy is uniform and the coverage of FFAs on NiFe2O4is monolayered withQovalue of 144.928 L·kg-1andr2value of 1.000.Data were further subjected to Freundlich isotherm model,which describes sorption process on heterogeneous surface as an exponential distribution of active sites and their energies[37]that can be considered as a multilayer sorption.It can be expressed as:

From the expression,qe(L·kg-1)is the amount of FFAs adsorbed at equilibrium on NiFe2O4,Kfrepresents the Freundlich isotherm constant,nis the adsorption intensity andCe(g·kg-1)is the equilibrium amount of FFAs.The plot of lnCeagainst lnqegave a straight line withr2value of 0.970,which suggest surface heterogeneity of NiFe2O4indicating that the surface of NiFe2O4may be heterogeneous in its approach for the sorption of FFAs.1/nas shown in Table 3 is a parameter that reflects the adsorption strength.When 1/n=1,then it means that the process is independent of the amounts of FFAs,when 1/n＜1,then the sorption is normal and when 1/n＞1 it shows a cooperative adsorption process.For the sorption of FFAs on NiFe2O4,value obtained for 1/nfalls within 1/n＜1,which shows that the process is a normal adsorption process.Previous study had shown that when value of 1/nis small,the heterogeneity becomes more feasible [38];this further supports the fact that heterogeneity might be feasible for the sorption of FFAs on NiFe2O4,since the 1/nvalue is 0.165.

Fig.4. Effect of temperature on adsorption capacity (a),percentage removal of FFA (b) and desorption capacity of NiFe2O4 (c).

3.5.Thermodynamics of adsorption

Effect of temperature on the sorption of FFAs by NiFe2O4was evaluated at temperatures ranging from 303 to 323 K and the results are presented in Fig.4a.The adsorption capacity increased with increase in temperature and amounts of FFAs.Similar observation was noticed for percentage removal as shown in Fig.4b.It can be inferred that temperature played a role in the sorption of FFAs on NiFe2O4.Parameters such as Gibb’s free energy change(ΔGo),enthalpy change (ΔHo) and entropy change (ΔSo) were determined.Sorption equilibrium constantbocan be expressed as [39]:

Fig.5. Electronic properties of FFA:Optimized geometry of C18:2 (a),HOMO density distribution of C18:2 (b),LUMO density distribution of C18:2 (c),Electron density of C18:2 (d),Electrostatic potential map of C18:2 (e),Local ionization potential map of C18:2 (f),Mulliken charge of C18:2 (g),Electrostatic charge of C18:2 (h).

Table 4 Effect of temperature on ΔG for the removal of FFA by NiFe2O4

Table 5 Molecular properties of FFA

whereCoandCeare initial and equilibrium amounts of FFAs,respectively,Ris the universal gas constant(8.314 J·mol-1·K-1)andT(K)is the absolute temperature.The plot of lnboagainst 1/Tgave a straight line from which ΔHoand ΔSowere determined from the slope and intercept.

The values obtained for ΔGoat the different temperatures(303-323 K) are presented in Table 4.The values are negative,which suggests that the sorption of FFAs on NiFe2O4is spontaneous.ΔHowas found to be 11.251 kJ·mol-1while ΔSowas 0.038 kJ·mol-1·K-1.The positive nature of ΔHoshows that the process is endothermic and the positive nature of ΔSosuggests high disorderliness between FFAs and surface of NiFe2O4,which indicates that sorption of FFAs on NiFe2O4was highly favored.

3.6.Desorption

Desorption study was conducted in order to evaluate the reusability of NiFe2O4,which guides for regeneration and subsequent reuse of NiFe2O4for the removal of FFAs in deteriorated vegetable oils.To achieve this,different solvent systems were used based on solubility of fatty acids.Fig.4c presents the desorption capacity of NiFe2O4for FFAs.The highest desorption capacity was 92%for mixture of chloroform:methanol(1:2)while the least desorption capacity (25%) was obtained for distilled water.Study has shown that chloroform/methanol mixture is a good mixed solvent for extraction of total lipids[40].This reveals it can be a promising solvent for the regeneration of NiFe2O4for further reuse as adsorbent for the removal of FFAs from deteriorated vegetable oils.

3.7.Quantum chemical computations

The interaction between the surface of NiFe2O4and FFAs was evaluated using quantum chemical computation involving the use of DFT performed at B3LYP/6-31G level theory using Spartan 14.1 software [41].Since C18:2 fatty acid was the most adsorbed fatty acid by NiFe2O4as shown in Table 1,the electronic properties of C18:2 was leveraged on in order to describe the interaction between the adsorbed FFAs and the surface of NiFe2O4.The electronic properties are presented in Fig.5.

Optimized geometry of C18:2 (Fig.5a) showed the presence of oxygen as a heteroatom in its molecule.Fatty acids generally has oxygen in their molecules.However,C18:2 and other unsaturated fatty acids such as C14:1,C16:1,C18:1 and C18:3 adsorbed by NiFe2O4contains double bonds,which are rich in electrons(π electrons).The presence of oxygen atom and double bonds in FFAs is an indication that FFAs contains non-bonding electrons that may be donated to the d-orbitals of NiFe2O4resulting in chemical bonding for the adsorption process.The non-bonding electrons are responsible for the negative Mulliken charges and the negative electrostatic charges.These negative charges further shows that FFAs is capable of donating electrons to NiFe2O4,which may be described as a donor-acceptor interaction.The molecular properties such as solvation energy (-11.28 kJ·mol-1),molecular surface area(3.93 × 1022m2) and dipole moment (1.53 debye) are shown in Table 5.

Fig.6. Molecular orbitals of FFAs.

The concept of donor-acceptor relationship is in agreement with the fact that the HOMO of FFAs possesses high electron density that could be donated for interaction with NiFe2O4.This interaction can be viewed as described in Fig.6,which shows the molecular orbitals of FFAs indicating electronic delocalization over oxygen,which reveals the ability of FFAs to transfer non-bonding electrons for interaction with NiFe2O4.

4.Conclusions

The study investigated the removal of FFAs from deteriorated vegetable oil using NiFe2O4.The NiFe2O4was synthesized via coprecipitation method.NiFe2O4was characterized using FTIR,XRD,TG analysis,BET,TEM,SEM and EDX.The removal of FFAs by NiFe2-O4revealed an adsorption capacity of 145.20 L·kg-1and a percentage removal of 80.67%.The characterization confirmed the synthesis of NiFe2O4with BET surface are of 16.30 m2·g-1being a type II BET isotherm.The sorption process can be described by pseudo-second-order kinetic model,which fitted best for Langmuir isotherm.Values obtained for ΔG,ΔHand ΔSshowed that the process is spontaneous and exothermic.The quantum chemical analysis revealed the mechanism of removal of FFA by NiFe2O4to be via donor-acceptor interaction.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

First and corresponding authors are grateful to the Department of Chemical Sciences,Redeemer’s University,Nigeria for provision of research space and chemicals.