1 Introduction

The dwindling fossil fuels and the stringent environmental regulations have led to the global exploration of renewable fuels, including biodiesel. Biodiesel is produced by trans-esterifying lipids from tree oils or microbial sources. Non-edible oils produced from several plant species including Pongamia pinnata, Jatropha curcas, Camalina sativa, Saussurea heteromalla, Cannabis sativa, and Toona ciliata are proved to be promising sources for producing biodiesel [1,2,3,4,5]. Several species of oleaginous yeast and microalgae also produce lipids sustainably that are suitable for biodiesel production [6,7,8]. In contrast, biomass can also be processed through various biological, thermal, and chemical processes to produce renewable hydrocarbon biofuels, including renewable diesel (RD) and sustainable aviation fuel (SAF) [9]. Any kind of alternate fuel can reduce net CO2 emissions in road and air transportation compared to conventional diesel and jet fuels and achieve a carbon–neutral economy. As per the federal policy of the Ministry of New and Renewable Energy (MNRE), India aims to blend 5% of biodiesel with petro-diesel. Nevertheless, feedstock’s availability and bioprocess’s economic viability pose a major challenge for the biofuel sector. Exploring technologies for sustainable resource valorization in a biorefinery framework will facilitate the development of a biomass-based circular economy.

Hydrotreating, gasification, catalytic conversion of sugars, pyrolysis, and hydrothermal liquefaction are promising technologies to process biomass into renewable biofuel. Hydrothermal liquefaction (HTL) is a sustainable, thermo-chemical conversion technology that operates at subcritical water conditions and transforms the organic biomass into different products in the presence of inert or reducing gases while oxygen is absent [10, 11]. The conversion of biomass occurs with deionized water as a solvent at 250–400 °C and 100–200 bar pressure. The efficiency of the HTL process depends on the reaction volume, pressure, temperature, catalyst, and initial feedstock constitution. During the hydrothermal treatment, the cellular components of biomass, including cellulose/starch, amino acids, and fatty acids, will be hydrolyzed into simple monomers [12, 13]. The products obtained from the HTL reaction, including bio-oil, aqueous fraction, and biochar, would possess several downstream applications, ensuring the overall sustainability of the bioprocess.

HTL successfully produced bio-oil from the oilseed of Litsea cubeba by optimizing temperatures and holding times [14]. Barley straw and biomass from different microalgae species were among the various feedstocks that were shown to be promising to produce value-added bio-oil at 280–320 °C using HTL [15, 16]. Depending on the reaction conditions and feedstock composition, 24 to 64% of the biomass could be converted into bio-oil which possesses a wide range of higher heating values (HHV) (calorific value indicator) of 28 to 38 MJ kg−1. Bio-oil is a black, sticky liquid with a rich density of hydrocarbons and serves as a resource for renewable diesel and SAF upon timely processing. On the other side, the solid fraction can be used as biochar with diverse functions, including enhancing soil quality by enriching soil C and as a solid fuel in boilers. Several studies have proven the increase in crop productivity after utilizing biochar as a soil enhancer. The application of biochar as a catalyst in transesterification will be a novel and green approach to valorize biochar and making biodiesel production more sustainable. Also, the aqueous fraction of HTL could be a valuable resource for microbial growth as it is rich in C and N sources. Though the bio-oil produced from the HTL reaction is well characterized and valorized, the biochar and aqueous components are far from characterization for their terminal applications. The presence of O, S, and N in the bio-oil will be disadvantageous for biofuel production and the use of catalyst in the reaction facilitates the removal of heteroatomic atoms along with an increase in product yields.

Amidst food security concerns and CO2 mitigation demands, the tree species Pongamia pinnata has emerged as a potential second-generation biofuel feedstock. It is native to subtropical climates and can grow under wider agro-climatic conditions. Pongamia has several advantages, including nitrogen fixation, high yield potential, and higher oxidation stability in biodiesel [17, 18]. Furthermore, Pongamia biodiesel provides much-improved braking thermal efficiency while minimizing the production of pollutants when compared to other oils [19]. Recent advancements in physiological and molecular studies lead to the better understanding of genomic and transcriptomic tools of Pongamia [18, 20]. Though the biodiesel produced from Pongamia oil is promising, the bioprocess is not economically viable due to the generation of futile by-products. The effective utilization of vegetative tissues, seed husks, and deoiled seed mass in Pongamia would create significant value addition beyond biodiesel production. Furthermore, these tissues are rich in unsaturated fatty acids and triacylglycerol, which are valuable sources of short-chain hydrocarbons and platform chemicals. The current study aims to develop an effective thermo-chemical conversion process to valorize the unutilized tissues of Pongamia for renewable fuel production. Deoiled seed biomass and seed husk along with oiled seeds were treated with hydrothermal liquefaction reaction and obtained diverse products which were further analysed for their composition. The unique compounds obtained out of HTL process corroborate the uniqueness of Pongamia towards biofuel production and anti-microbial and medicinal properties. Also, the current study highlights the utilization of aqueous fraction of HTL to grow oleaginous yeast species.

2 Materials and methods

2.1 Feedstock and reagents

Pongamia pods used for oil extraction were obtained from Tree Oils India Limited (TOIL), Zaheerabad, Telangana state, India. To extract oil, Pongamia seeds were finely ground and then loaded onto a Soxhlet extractor with the help of a thimble as described previously [17]. The extractor is fitted with a 500-mL round-bottomed orbital flask, and condenser and the extractions were carried out on a heating mantle for 6 h with excess n-hexane. The solvent was separated under vacuum in a rotary evaporator (Hei-VAP Core; Heidolph, Germany) at 65 °C, and the extracted oil was used for downstream applications. Furthermore, Pongamia seed husk (PpH), raw seeds (PpO), and deoiled seeds (PpD) were processed through HTL in the presence of water. The reactions of seed husk, raw seeds, and deoiled seed tissues were carried in the absence (represented as PpH, PpO, and PpD respectively) and the presence (represented as PpHC, PpOC, and PpDC respectively) of the Ruthenium on carbon (2%, Ru/C) catalyst. All the solvents and chemicals including n-hexane, sodium hydroxide, cyclohexane, hydrosulfuric acid, hexane, potassium hydroxide, diethyl ether, methanol, toluene, potassium carbonate, and chloroform were of analytical grade from Merck and Sigma-Aldrich.

2.2 Hydrothermal liquefaction of Pongamia feedstock

Hydrothermal reaction was performed in a custom-designed laboratory-scale high-pressure stirred reactor (KLB Instruments Ltd., India). The main vessel was made of C276 alloy (Hastalloy), and other components of the system were made of SS316. The internal diameter and depth of the reactor were 64 and 100 mm with a length and depth ratio of 1.56 and total capacity of 300 mL respectively. The vessel is detachable from the fixed reactor head which was fitted with a magnetic drive (16 kgf cm torque) mounted on 9/16 UNF port with gasket. Also, three gas inlets with valves and dip tube were connected to the vessel head to sparge three different gases simultaneously and the gas vents (¼″) were connected to blowout tank (3 L) with a sample drain port. The blowout vessel assists in reactor safety, gas collection, and regulated release of vapour upon its neutralization. An internal cooling loop was fitted and connected to water supply for instantaneous cooling of the reactor after set time and to protect the components from excessive heat during the reaction [16]. A ceramic heater with swinging movement is fixed and connected with a type K thermocouple which assist in preventing any possibility of heater burnout. A PID (proportional-integral-derivative) controller with four slots for real-time rpm, reaction pressure and temperature are attached to the reactor. Heater cut-off was incorporated to failsafe any temperature overshoot. The reactor mounted on a heavy-duty base plate can withstand working pressure of 200 bar and operating temperature of 350 °C. The experimental setup and corresponding product yields are represented in Fig. 1. 

Fig. 1
figure 1

Schematic representation of HTL experimental setup and the corresponding yields obtained from different tissues of Pongamia

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Deionized water (200 mL) was used as solvent for HTL reaction of Pongamia feedstock (15% w/v) in the presence and the absence of catalyst (2% w/w of feedstock). Initially, the reactor headspace was purged with nitrogen gas (N2) up to 40 bar to replace the air and to create inert atmosphere. After attaining the set temperature of 250 °C, the HTL reaction was carried out for 1 h. Upon completion of the reaction, the reactor was brought to ambient temperature by passing cold water through the cooling coils thus stopping the further reaction. The gases (HTLGas) in the headspace were collected into gas bags for subsequent gas profiling. Solid–liquid fractions of reaction mixture were separated with a filter paper using a vacuum/suction pump. The solid part was washed with chloroform for several times to recover organic-soluble residues, and the separated solid residue was dried at 105 °C for 12 h and weighed [16]. For catalytic HTL samples, the weight of the biochar (HTLBc) was measured after subtracting the initial catalyst weight. Biochar yield was calculated by using Eq. (1).

$$\mathrm{Biochar yield}\left(\%\right)=\left(\mathrm{Weight of Biochar}/\mathrm{Weight of Feedstock}\right)\times 100$$
(1)

Organic biocrude and aqueous (HTLAq) fractions were separated by liquid–liquid separation using acetone/chloroform for further characterization. High heating value (HVV) that is an indicator of calorific value of biochar was calculated by using Eq. (2) [21]

$$\mathrm{HVV}\left({\mathrm{MJ kg}}^{-1}\right)=0.3383\mathrm{C}+1.442\times \left(\mathrm{H}-O/8\right)$$
(2)

2.3 Analysis

Gas chromatography–mass spectrometry

Bio-oil generated from HTL reaction was analysed using Gas Chromatography-Mass Spectrometer (GCMS; Agilent). The chromatographic peaks were obtained by using a capillary column (HP-5MS, 30 m × 0.25 µm × 0.25 µm) using helium as the carrier gas. The column temperature was set at gradient of 50–280 °C at a rate of 10 °C/min. The injection chamber was maintained at 280 °C with injecting size of 1 µL [22]. NIST mass spectral database was used to identify individual peaks of GC–MS.

Fourier transform infrared spectroscopy

HTLBc (2 mg) obtained from different HTL reactions were added with potassium bromide (KBr) (200 mg). After preparing the contents into a homogeneous mixture, sample discs were prepared by pressing with the help of a hydraulic press for 5 min. Each sample was measured over the infrared spectra in the range of 400 to 4000 cm−1 for 64 scans at a resolution of 4 cm−1 using an FTIR spectrometer (PerkinElmer). Control discs of KBr without sample were used for background correction.

High-resolution mass spectrometry

The composition of the aqueous phase of the HTL reaction was determined using a Quadrapole Time-of-Flight Mass Spectrometer (QTOF; Waters AcquityXevo G2-XS) equipped with a C18 column (Aquity CSH) having 2 mm × 100 mm and with a 1.7-µm particle size (Waters, Milford). The flow rate was set at 0.3 mL/min in the presence of 0.1% formic acid as mobile phase in water (solvent A) and acetonitrile (solvent B) at 25 °C. The gradient elution was done for all the samples to identify aqueous soluble compounds as described [23].

Gas chromatography

The resultant gases obtained from HTL reactor (HTLGas) were collected in gas sampling bag and analysed for their composition using gas chromatography (GC; Agilent–7890B) thermal conductivity detector (TCD). Hey-Sep Q column (1/800 × 2 m) was used with argon as carrier gas. The injector and detector were run at 80 °C, and the oven was maintained at 100 °C isothermally [10].

Elemental (CHNSO)

The elemental composition of Pongamia biochar (solid sediment fraction) was determined using organic elemental analyser (Vario Micro cube, Elementar). Sulfanilamide was used as CHNS reference, and benzoic acid was used as oxygen reference. A pressure of 1250–1300 mbar was maintained using helium and oxygen as carrier gases at a flow rate of 200 and 10 mL/min respectively (only helium gas for oxygen). Vario Micro software was used for data acquisition.

2.4 Yeast growth and biomass production using HTL Aq

Trichosporon cutaneum (NCIM 3326) cultures were maintained in yeast specific medium (0.3% yeast extracts; 0.3% malt extracts; 0.5% peptone; 1% glucose). For batch fermentation, 250 mL of MGYP medium and HTLAq broths were inoculated independently with 10% of overnight grown yeast cultures. The growth, biomass, and lipid quantification of the yeast cultures were monitored and measured according to [24].

3 Results and discussion

3.1 Product yields from HTL reaction

Different tissues of Pongamia pinnata including deoiled seed (PpD/PpDC), oiled seed (PpO/PpOC), and seed husk (PpH/ PpHC), showed relatively different product yields upon treating with HTL reaction in the presence of Ru/C catalyst (Table 1). Temperature, heating rate and reaction holding time are the most critical factors in the HTL process impacting the product portfolio and composition. Subcritical temperature of water especially in the range of 250 to 300 °C is considered optimal, and reaction time of 30 to 90 min holding will give better product yields in the case of majority of biomass. Poor bio-oil production was generally noticed from insufficient breakdown and polymerization, whereas longer reaction durations will cause polymerization between intermediate products [12, 25, 26]. In the current study, Pongamia tissues underwent liquefaction for 60 min after reaching the set temperature and the pressure of the reactor increased gradually from 40 bar during the reaction and the final pressures vary among the samples depending on their molecular composition (Table 1). It was observed that the terminal pressure of the reaction was lower in catalytic HTL when compared to their non-catalytic counterparts indicating catalyst’s role in reducing the activation energy of the HTL reaction. A maximum pressure of 111 bar was observed in PpD sample, while the PpOC sample witnessed a minimum pressure of 82 bar (Table 1). During the hydrothermal treatment under subcritical water, each biomolecule transforms through specific chemical reactions. For instance, monosaccharides undergo oxidation or condensation to form polar intermediates such as carboxylic acids, aldehydes, and ketones. In contrast, fatty acids undergo decarboxylation to yield ketones, alkanes, and alkenes. Amino acids undergo deamination to yield organic acids and ammonia, while decarboxylation generates amines and carbon dioxide [27, 28]. Hence, the feedstock composition will determine the final product and their composition. The initial feedstock weight of 20 g of different Pongamia tissues has shown varied product yields. Furthermore, there was a significant difference in product yields in the presence of a catalyst. Interestingly, HTL of Pongamia yielded higher amounts of biochar contents both in the presence and in the absence of a catalyst (Table 1). Maximum biochar was obtained with PpD tissues which were recorded as 48.2%. Catalyst usage has reduced biochar and increased the bio-oil yield in all the tissues of Pongamia. The bio-oil ranges from 32 to 46% in Pongamia, whereas the aqueous fraction ranges from 15 to 21% with considerable change in the presence of a catalyst. Catalysts play a key role in HTL’s product specificity and heteroatom reduction (O, N, and S) through hydrothermal deoxygenation. Especially, relatively inexpensive and highly active heterogenous catalysts including transition metals (Ru, Pt, Mo, and Ni) and zeolites showed improved efficiency concerning bio-oil yields and catalyst recovery in lignocellulose and microalgae feedstock. At the same time, other studies have reported an increase in high heating value (HHV) and bio-oil yield by using homogeneous catalysts [29]. Ru/C catalyst used in the present study has significantly impacted the Pongamia HTL reaction’s product yield and showed increased bio-oil production. Recycling the catalyst by washing it with organic polar solvents, surfactants, and chelators and calcination at a high temperature could be a more sustainable approach when working with HTL bioprocessing [30, 31].

Table 1 Reaction conditions and productivities resulting from HTL reactions of different Pongamia tissues
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3.2 HTL derivatives

3.2.1 Aqueous fraction (HTL Aq )

The aqueous fraction obtained from HTL reaction was analysed with HRMS to understand its composition. The data broadly revealed the presence of sugars and sugar derivatives, fatty acids, esters, and phenolic compounds (Supplementary Table 1). The major product categories that are produced into HTLAq are saccharides, alcohols, volatile fatty acids (VFA), hydrocarbons, cyclopentenones, phenols and phenol derivatives, amino acids, ammonia and melanoidins, N-heterocyclic compounds, oxygenates, esters, and ketones (Supplementary Table 1). There is a clear difference between products obtained from seed husk and seed as the chemical composition of both tissues varies. Furthermore, the deoiled seed is devoid of fatty acids and hence the lipid derivatives represented in the aqueous fraction of PpD and PpDC were low compared to PpO and PpOC. Few compounds representing plant metabolites and natural products including diterpenoids were present in the aqueous fractions. For example, kobusone, diglycerol, syringic acid, vanillyl mandelate, roughanic acid, and cladinose are identified in the HTLAq samples of Pongamia tissues (Supplementary Table 1). In general, the intermediates of metabolic pathways and end products of biochemical reactions and secondary polymeric compounds that are water soluble can be found in the aqueous fraction of HTL. Depending on the reaction conditions, the organic material present in diverse feedstock showed a conversion efficiency of 20–50% into aqueous fraction during the HTL reaction [32]. The chemical structure of biomass is substantial in determining the reaction pathways and their aqueous products [33, 34]. The compounds obtained in the aqueous fraction of Pongamia possess various end use applications corroborating the Pongamia’s anti-microbial and medicinal properties in addition to its application as biofuel feedstock [35, 36]. Triglyme and other organic compounds in HTLAq show the adhesive and emulsion properties of Pongamia seed. Fatty acids including myristic acid (C14), caprylic acid (C8), lauric acid, acetic acid, and crotonic acid are evident in the HTLAq fraction of Pongamia (Supplementary Table 1). In general, hemicellulose will be decomposed into soluble fragments and low molecular weight xylooligomers, glucose, and mannose and finally degraded into furanic compounds. Lignin is majorly degraded into phenol, methoxyphenol, and catechol during HTL [37]. Levulinic acid, a cellulose breakdown product, that acts as a biofuel precursor was found in Pongamia seed husk and deoiled samples (Supplementary Table 1). The absence of levulinic acid in oiled seed could be due to the different chemical reactions and feedstock composition. Lipids can be degraded into fatty acids and glycerol which are further degraded into formaldehyde, acetaldehydes, and alcohol. Short-chain fatty acids and volatile carboxylic acids in HTLAq of Pongamia lipid derivatives could act as a substrate for microbial growth. In other similar experiments, woody biomass containing major portions of cellulose, hemicellulose, and lignin yielded phenols, furfurals, glycolic, acetic acid, alcohols, and cyclopentenones [38, 39]. Orange pomace in which cellulose is a major fraction gave acetic acid, HMF, furfurals, ethanol, acetone, butanone, and alkyl derivatives [40]. With significant quantities of sugars, carboxylic acids, and VFAs, the HTLAq from Pongamia could be a potential resource for value addition and resource recovery.

3.2.2 Organic fraction (biocrude/bio-oil)

GC–MS was used to analyse the volatile compounds of bio-oils after extracting with ethyl acetate (Supplementary Fig. 1). Table 2 represents the identified compounds, relative peak area (%), and retention time (RT) among all the GC/MS detectable species. The GC–MS data indicated the presence of furans, aldehydes, cyclopentenones, ketones, alcohols, and phenolics along with aliphatic hydrocarbons in HTL-derived Pongamia bio-oil. There is a quantitative and qualitative difference in the bio-oil among the tissues and in the presence of a catalyst. Organic compounds ranging from C4 to C19 were observed, whereas the proportion of heteroatomic compounds including N and S was relatively low in all the samples. Among the N-containing organic compounds, in considerable concentrations, methylpyrazine, dimethylpyrazine, dimethylpyrimidine, and ammonium benzoate were present (Table 2). PpH and PpHC samples showed unique phenolic compounds including syringol, phenol, and hydroxyl-3-methoxyphenyl acetone compared to the other two samples (Table 2). The fatty acids such as stearic acid, levulinic acid, myristic acid, and palmitic acid were identified in bio-oil obtained from PpO and PpOC. The use of catalyst favoured the occurrence of cyclopentenones and reduced the prevalence of heteroatomic compounds as evidenced by the results of PpDC, PpOC, and PpHC (Table 2). Furthermore, catalyst favoured the formation of aliphatic hydrocarbons in bio-oil indicating more C contents, thus making them more suitable as fuels. HHV value depends on a high H/C ratio which in turn depends on the decarboxylation and dehydration of aldehydes, acids, alcohols, and ketones, thus forming hydrocarbons. Most importantly, fatty acid decarboxylation into alkane during HTL in the presence of alkali and metal oxide catalysts contributes to the increase in the H/C ratio. Pongamia seed being a biofuel feedstock is rich in unsaturated fatty acids and could be an ideal source of bio-oil with high calorific value. Besides decarboxylation, the deoxygenation through dehydration and hydrogenation also contributes to bio-oil’s thermal stability. In particular, phenol undergoes hydrogenation and dehydration to form cyclohexane during HTL. Metal oxide catalyst such as Ru/C used in the current study is known to deoxygenate phenol into cyclohexanes. Removal of nitrogen and sulfur through denitrogenation and desulfurization is necessary to improve bio-oil quality as they lead to undesirable NO2 and SO2 emissions. Bio-oil obtained from several other biomass feedstocks such as Cymbopogon citratus was blended with petro-diesel with an efficient fuel efficiencies [41]. Value-added compounds produced through the HTL reaction of Pongamia can be further valorized by using hydrotreating approaches, wherein the bio-oil could be a potential source to produce sustainable aviation fuels as it showed aliphatic hydrocarbons and rich fatty acid derivatives and esters.

Table 2 Diverse compounds and their retention times along with area percentage obtained through GC–MS analysis of bio-oil generated from HTL reactions of different Pongamia tissues
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3.2.3 Gaseous fraction (HTL Gas )

Gas profiling of the mixed gases collected from the reactor head of the HTL reaction vessel was performed using gas chromatography. Hydrogen (H2), methane (CH4), and carbon dioxide (CO2) were generated predominantly in the reactor upon hydrothermal treatment of Pongamia seed (Fig. 2A). There was a significant difference between the catalytic and non-catalytic HTL outcomes, wherein the use of catalyst increased H2 and decreased CO2 concentrations in all the samples. The H2 content was recorded as 23.6, 19.9, and 20.1% in PpH, PpO, and PpD samples respectively which increased to 40.3, 32.5, and 41.5% in corresponding catalytic HTL reactions (Fig. 2A). Conversely, the CO2 levels were decreased in the presence of the catalyst and recorded as 59.4%, 48.6%, and 45.7% in PpHC, PpOC, and PpDC samples respectively. Catalysts are selective in mediating chemical reactions, especially during thermo-chemical conversions. For instance, nickel-based catalysts are known to carry out CO2 methanation at relatively low temperatures, thus converting excess CO2 and H2 into CH4. Optimizing HTL conditions to achieve the balance between H2 and CO2 generation can improve the overall efficiency of the process. The results of the current study corroborate the fact that the biomass components underwent hydrogenation and catalytic hydrolysis in the presence of the transition metal catalyst and produced enhanced H2. Maximizing the production of liquid hydrocarbons, while minimizing the formation of CO2, will improve the downstream applications of HTL to produce biofuels or other valuable chemicals. The CH4 contents were relatively low when compared to the other two gases and noticed that its content was further decreased in the presence of catalyst in all the samples (Fig. 2A). In general, the catalysts show varied responses in quantity and quality of gases that are generated from HTL reaction with different feedstocks. HTL of microalgae biomass in the presence of alkali catalysts released higher CO2, whereas organic acid–based catalysts resulted in a rise in the amount of H2, CO, and CH4 in gas products from similar biomass [42]. The behaviour of catalyst also depends on the initial supply of H2, wherein the presence of H2 shows increased production of CO2. The use of Pt/C catalyst led to a rise in the C2H6 and CH4 content from microalgae biomass, while Ni/SiO2-Al2O3 and Ru/C catalysts demonstrated a better decarboxylation activity, thus increasing C2H6, CH4, and H2 production [43]. Usually, the H2 is utilized to remove O2 from the feedstock by forming H2O vapour which in turn facilitates pressure build-up during HTL reaction. In Pongamia, the decrease in CO2 levels in the presence of a catalyst corroborates the fact that the Ru catalyst enhanced the conversion rates of feedstock in HTL reaction into either bio-oil or biochar, thus reducing the gas formation.

Fig. 2
figure 2

A Profiling of gaseous fraction obtained from HTL reaction of different Pongamia tissues in the presence and absence of catalyst through gas chromatography. B Elemental analysis of biochar obtained from HTL reaction of different Pongamia tissues in the presence and absence of catalyst through CHNS analysis

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3.2.4 Solid fraction (biochar-HTL Bc )

The hydrothermal degradation of Pongamia seed has led to the formation of a rigid amorphous carbon matrix residue of biochar upon washing the slurry with organic solvents followed by the separation of the volatiles and aqueous fractions. The structural chemistry of feedstock influences the yield and composition of biochars. Elemental analysis of raw seeds and HTL-derived biochar was performed to understand the chemical composition and HHV of Pongamia biochar. The carbon content of deoiled seed was significantly lower when compared to unprocessed seed and seed husk. In contrast, there is no significant difference between catalytic and non-catalytic biochar composition. The C content of the oiled, deoiled, raw seed, and seed husks is lower when compared to their corresponding biochar. After oil extraction the oxygen content of the deoiled seed reduced significantly which was reflected in subsequent biochar, indicating minimal oxygen-containing functional groups in deoiled samples. The HVV which indicates the calorific value of the biochar was calculated based on the elemental composition and found that biochar showed significantly higher HVV when compared to the original biomass (Fig. 3A). Among biochar from different tissues, HTLBc from PpOC showed a superior HVV index compared to other tissues. It was evident from the data that catalyst usage has little impact on the HVV of the biochar wherein there was no significant change between the two reaction sets. The O/C and H/C of biochar indicate the polarity and aromaticity of biochar and influence its stability and extent of charring and applicability as an adsorbent (Fig. 3B).

The FTIR spectra of biochar obtained from different samples are shown in Fig. 3C, while Table 3 presents the possible compounds and the corresponding functional groups of different biochar samples. The structural group of the obtained biochar was assigned by following the published FTIR spectra in standard FTIR libraries [44]. FTIR spectrum of normal and catalytic HTL-derived biochar is relatively similar; however, the strength of the absorption bands varies between the two groups. Furthermore, there was a difference in the band pattern of individual tissues, indicating the different chemical natures of selected tissues. The aliphatic hydrocarbons including alkanes, alkenes, and alkynes are predominant in all the samples spreading across different wavenumbers. The C = O functional group corresponding to 1650 to 1800 cm−1 wavenumber is absent in deoiled seed indicating the removal of carboxylic acids. Interestingly, the C = O functional group is present in seed husk-derived biochar indicating the presence of lipid content. The results correlate with elemental analysis, wherein the deoiled seed-derived biochar has less carbon content when compared to PpO and PpH. However, the deoiled seed-derived biochar showed the presence of aliphatic amine groups at 3412 and 3336 cm−1 in non-catalytic and catalytic HTL respectively. As a result, the elemental N in deoiled seed-derived biochar is more when compared to its counterparts. The FTIR spectrum also revealed the presence of halo compounds in all the samples showing the characteristic feature of Pongamia. Another noticeable feature is the presence of –OH stretch in PpO and PpOC biochar which supports the presence of alcohol or phenolic functional groups. Oxygen is associated with biomass’s hydroxyls, phenols, ethers, carbonyls, and carboxyl functional groups. In contrast, hydrogen is associated with surface functional groups, aliphatic compounds, and the surfaces of aromatic structures (aromatic C-H). During hydrothermal treatment, the structural H and O are lost primarily as H2O, while C is condensed into aromatic structures.

Fig. 3
figure 3

A HVV values calculated from elemental profile, representing the calorific value of biochar obtained from HTL reaction of different Pongamia tissues in the presence and absence of catalyst. B H/C and O/C ratio of biochar obtained from HTL reaction of different Pongamia tissues in the presence and absence of catalyst. C FTIR absorption peaks in different biochar samples obtained from HTL reaction of different Pongamia tissues in the presence and absence of catalyst

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Table 3 FTIR analysis of biochar samples obtained through HTL reaction and band positions along with possible functional groups in Pongamia samples
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3.3 Recycling of HTL Aq for yeast growth towards circular bioeconomy

As the HTLAq possesses toxic and inhibitory substances including furans and phenols, it is unsafe to release the fractions untreated into the external environment. It might be risky to marine life and negatively influence the fertility of land and water thus deteriorating the environment. In general, the aqueous fractions of HTL reaction of lignocellulosic biomass possess a chemical oxygen demand (COD) of 40–110 g/L, which goes up to 160 g/L for non-lignocellulosic biomass. Furthermore, the VFA and carboxylic acids derived from lignocellulose biomass make the aqueous fraction acidic with a pH range of 3.4 to 6.2 making it less suitable for aquatic or terrestrial life. Hence, dissolved organic carbon and nutrients in the HTLAq could be reduced through complimentary accessory processes that produce useful products, thus making HTL more economical for future deployment. Bioprocesses including anaerobic digestion (AD), algae cultivation, yeast fermentation, and supercritical water gasification showed promising results in recovering resources for valuable compounds from HTLAq before discharging into the environment. However, the presence of toxic and refractory compounds in HTLAq limited the efficiency of AD and algal cultivation [45]. In the current study, it was evident from the HRMS analysis that the HTLAq is rich in sugars and VFAs which could be a rich resource for microbial growth. Trichosporon cutaneum is a non-conventional oleaginous yeast species known for producing high lipid contents by tolerating the inhibitory substances present in the waste resources [7]. Tc was cultured at 25 °C for 96 h using synthetic media (MGYP) and processed HTLAq, and growth kinetics and biomass accumulation were measured at regular intervals. Usually, pH plays a crucial role in growth kinetics and lipid accumulation of oleaginous yeast species, wherein a pH range of 7.0 to 8.0 was shown to be optimum for most of the oleaginous yeast species. The COD of HTLAq samples from various tissues of Pongamia ranges from 89 to 102 g/L which was diluted to 15 g/L, and the pH was adjusted to 6.5 before the inoculation of yeast. The pH of the cultures increased along with the absorbance as the growth progressed in all the samples. The maximum final biomass accumulation and lipid productivities of Tc at the end of the growth period were recorded as 2.94 g/L and 29.6% (Fig. 4). The biomass accumulation and lipid yields were lower when compared to the synthetic medium which is presumably due to the mild effect of inhibitory substances present in the renewable feedstock (Fig. 4). Previous studies showed the efficacy of Tc in valorizing the spentwash by utilizing it as renewable feedstock [24]. Other yeast species including Lipomyces starkeyi, Rhodotorula toruloides, and Cryptococcus albidus were also successfully grown using renewable feedstock to produce lipids [6, 46]. Yarrowia lipolytica is also used as a promising host to valorize aqueous fractions of hydrothermal liquefaction to produce lipids, itaconic acid, and triacetic acid lactone [47]. Pongamia HTLAq fractions could also be a valuable resource for the growth of other microbes to produce value-added compounds in a circular economy format in a biorefinery approach.

Fig. 4
figure 4

Biomass and lipid productivities of Trichosporon cutaneum at the end of 96 h grown using aqueous fractions of HTL reaction from different Pongamia tissues in the presence and absence of catalyst

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4 Conclusions

The study outlined a sustainable hydro-technology for the valorization of Pongamia biomass into diverse products including bio-oil, biochar, biogas, and aqueous fractions. HTL of seed husk and oiled and deoiled seeds at 250 °C in the presence of N2 yielded more biochar, while the usage of Ru/C catalyst enhanced bio-oil and reduced CO2 evolution. The aqueous fractions, rich in sugars and VFA, supported the growth of oleaginous yeast, Tc, for lipid production. Compounds such as kobusone, syringic acid, roughanic acid, and cladinose revealed the value addition of Pongamia biomass. The data revealed the potential of bio-oil towards the production of SAF through hydrogenation due to the presence of rich amounts of carboxylic acids. The biochar could act as a potential catalyst in transesterification of lipids to produce FAMEs. Further studies can explore the potential of HTL by-products towards several downstream applications. In conclusion, the present study investigated the potential of Pongamia pinnata biomass to produce value-added compounds in a circular economy framework.