Valorization of lignocellulosic solid waste obtained from essential oil industry for bio-oil production and dye removal

Essential oils are vital components of life, renowned for their remarkable medicinal properties, including their ability to induce sleep and alleviate stress and anxiety through aromatherapy. After the extraction of essential oils from natural sources such as ginger, turmeric, pepper, fennel seed, and carrots, the residual solid materials are often disposed of in landfills or incinerated, resulting in environmental pollution (Sowbhagya 2019). It is anticipated that the market for extracted essential oils would expand at a CAGR of 7.4% between 2018 and 2028, resulting in a greater quantity of solid waste (Get News 2022). Consequently, there is an urgent need to explore methods for the value-added utilization of these spent solid residues to ensure both economic and environmental sustainability.

It is noteworthy that various spent solid wastes from the essential oil industry have been the subject of research for extracting value-added compounds, serving as adsorbents, and even for biofuel production (Kumar & Ahmad 2011; Alismaeel et al. 2022). However, no study has reported the complete valorization of carrot seed waste. A recent report has indicated that the market for carrot seed oil is expected to experience robust growth by 2029 owing to its popularity in aromatherapy and antimicrobial, antioxidant, and antifungal characteristics, especially in North America, Latin America, Europe, East Asia, South Asia, Oceania and Middle East, and Africa (Sieniawska et al. 2016). Carrot seeds contain only 10–15 wt% of essential oils resulting in around 85–90 wt% of carrot seed as the solid waste which requires proper disposal and management. Carrot seed waste can be utilized by two ways: (i) feedstock for bio-oil production and (ii) as an adsorbent (raw and biochar) for wastewater treatment. It is a suitable candidate for bio-oil production through slow pyrolysis due to its high lignocellulosic content that produces bio-oil, biochar, and little amount of syn gas (Yin et al. 2021). Bio-oil has versatile applications, including biodiesel and chemical production, while biochar is predominantly utilized for remediating contaminants in soil, water, and air due to its ample surface area, high porosity, functional groups, cation exchange capacity, and stability (Wang et al. 2023).

Nowadays, most textile industries are responsible for creating water pollution by discharging untreated dye-bearing effluent streams into water bodies. Methylene Blue (MB) dye is a commonly used dye in the textile industry that could be highly toxic to human health, causing tissue necrosis, vomiting, nausea metal, neural disorder, and methemoglobinemia (Abu-Dief et al. 2021b; Alahmadi et al. 2023). Several treatment processes (conventional and advanced), such as coagulation–flocculation, biological treatment, adsorption, oxidation, have been studied to treat dye-bearing wastewater (Mohamed & Abu-Dief 2018; Mohsen Alardhi et al. 2020; Ali et al. 2022b; Alzaid et al. 2023). Among all, biosorbent-based adsorption has garnered particular interest due to its effectiveness and eco-friendliness (Al-Jaaf et al. 2022). Agro-industrial waste, due to its affordability, widespread availability, and ease of use, presents favorable characteristics such as porous morphology, crystalline nature, and metal oxide content for effective adsorption applications (Al-Jaaf et al. 2022).

The primary objective of this study is to repurpose spent carrot seed waste (SCSW) for dual applications: (i) bio-oil production and (ii) the treatment of MB-containing wastewater. Initially, a comprehensive characterization of SCSW is carried out using techniques such as field emission-scanning electron microscopy (FE-SEM), Brunauer-Emmett-Teller (BET), proximate analysis, thermogravimetric analysis (TGA), and differential thermal analysis (DTG), Fourier transform infrared (FTIR), and ammonia-temperature programmed desorption (NH₃-TPD). Subsequently, SCSW undergoes pyrolysis at 500 °C in a fixed bed reactor to yield bio-oil and biochar. Additionally, the GC-MS analysis of the bio-oil is carried out to identify its composition and constituent compounds. Finally, to assess the efficacy of SCSW and its biochar as adsorbents, a batch adsorption study is performed for the removal of MB from wastewater. It is worth noting that this study represents a novel, comprehensive examination of the value-added utilization of SCSW, as no prior research has explored all these aspects simultaneously.

All the chemicals used in the present work were of analytical reagent (AR) grade. The MB powder (Q39692) and NaOH (97% purity) are procured from Thermo-Fisher Scientific; HCl (35%) is purchased from Rankem; KOH (86%) and chloroform (99.9%) are procured from Agnitio pharma. Orbital Shaking Incubator, UV-spectroscopy, and pH meter are procured from Labgear International, Labtronics (LT-2204) and Metrex auto deluxe, India, respectively. Double-distilled water is used throughout the analysis. The SCSW sample is procured from Rajeshwari Essential Oil Company, located in India and washed repeatedly with de-ionized (DI) water using a G2 funnel until a clear supernatant is achieved and then subjected to a drying process in a vacuum air oven at 80 °C, for a duration of 24 h to eliminate moisture. The dried SCSW sample was subsequently stored in an airtight container for further research.

BET surface area analysis is conducted using Autosorb 1 (Quanta chrome) with nitrogen gas to analyze adsorption–desorption isotherm at 77.35 K. The pH drift method is used for point-of-zero charge determination (Gadelha et al. 2023). In this method, 50 mL of 0.01 M NaCl is added to 10 conical flasks with pH adjusted from 3 to 12 using 0.1 M HCl and 0.1 M NaOH. Subsequently, 0.15 g of the SCSW sample is added to each conical flask, which is then sealed and placed in an orbital shaking incubator for 48 h at 30 °C to reach equilibrium. After 48 h, the filtrate is collected from each conical flask, and pH_final is measured using a pH meter. The graph is plotted between ΔpH (pH_final − pH_initial) vs. pH_initial to find the intersection point where ΔpH = 0 gives the pH_pzc of SCSW. The experiments are conducted in triplicates.

TPD analysis is carried out using Micrometrics Auto ChemII 2750 instrument. For this, 50 mg of the SCSW sample is loaded into a U-shaped quartz reactor and preheated at 200 °C for 2 h in the He atmosphere (20 mL/min). After cooling to room temperature, the sample is exposed to 10% ammonia in the He environment and heated at 10 °C/min for 60 min till adequate ammonia is adsorbed onto the SCSW surface. In the final step, the SCSW sample is heated in helium environment from 50 to 950 °C at a rate of 20 °C/ min for 90 min to obtain thermal conductivity detector (TCD) signals.

For FE-SEM analysis (JOEL7610F), SCSW samples are gold-coated prior to capture images at 900× and 1,500× magnification levels. FTIR analysis is performed using Nicolet iS20 FTIR Spectrometer (Thermo Scientific), where 10 mg of the sample is taken in ATR crystal, clamped using a pressure gauge, and placed in the FTIR instrument sample holder. The IR scanning is done from 500 to 4,000 cm⁻¹ range with a 1 cm⁻¹ gap and the scanning time is 20 s. TGA/DTG analysis is carried out using Thermogravimetric Analyzer (Mettler Toledo). Nine mg of the SCSW sample is isothermally held at 35 °C for 5 min, and then heated to 800 °C at a rate of 10°C/min. Nitrogen gas flows at a constant rate of 40 mL/min throughout the analysis.

Pyrolysis of SCSW is carried out using a thermochemical conversion setup, consisting of a fixed bed reactor (34 mm dia. and 410 mm height) housed within a furnace, as presented in Supplementary material, Figure S1. The reactor is insulated with glass wool, and the temperature of the heater is controlled using a proportional integral -derivative (PID) controller. An inert atmosphere is maintained using nitrogen with a flow rate of 100 mL/min. Fifty g of SCSW is fed to the reactor and heated to 500 °C with a rate of 10 °C/min for 60 min. In downstream of the setup, a condenser is deployed alongside a liquid–gas separator for the collection of the resulting bio-oil. Percentage gas yield is estimated by deducting the weight of bio-oil and biochar from the original feed mass (Kumari & Mohanty 2020). To identify the chemical compounds in bio-oil, GC-MS (Agilent 5977B GC/MSD) is used with HP-5MS capillary column (30m × 0.25 mm, 0.25 μm). The bio-oil sample is prepared by diluting with chloroform (HPLC-grade) in the ratio of 1:10 and then injected at a split ratio of 1:10. The initial temperature of the column is kept at 60 °C, and then the temperature is raised to 300 °C at a ramping rate of 2 °C/min. Helium is used as a carrier gas with a flow rate of 3 mL/min, and the compounds are identified using NIST library.

Biochar derived from the pyrolysis of SCSW is activated by following the procedure given by dos Santos et al. (2019). Ten g of SCSW biochar is taken in a 250-mL round bottom flask with 100 mL of 8 M KOH and connected to a reflux condenser. The mixture is heated in an oil bath at 80 °C for 2 h at 120 rpm and filtered, and then biochar is kept in an air oven for 12 h at 110 °C. Further thermal treatment of activated biochar (ABC) is done by heating the sample in the tubular furnace at a controlled heating rate of 10 °C/min to achieve the final temperature of 800 °C for 90 min. The sample is then cooled and washed several times with distilled water until the final pH of supernatant reaches 7. After washing, samples are kept in an air oven at 110 °C for 12 h.

From the ultimate analysis given in Table 1, the presence of high carbon content can be seen due to the organic attributes of SCSW. The carbon-containing lignocellulose could be helpful for the adsorption of organic compounds or heavy metals from the wastewater (Wang et al. 2023). H/C and O/C ratios are 1.7 and 0.75, respectively, which fall within the range of typical lignocellulosic biomass as per the Van Krevelen diagram. Nitrogen content is less, thus, low amount of nitrogen oxides will be produced during the thermochemical conversion process (Saikia & Bardalai 2018). The proximate analysis (Table 1) shows that the ash content is comparatively more than fixed carbon content due to the presence of metals and inorganic compounds, making SCSW different from typical lignocellulosic biomass. The presence of high volatile content (71.3%) in SCSW is comparable to other conventional lignocellulosic biomass, such as brewer's spent gram, having a volatile content of 74% (Saikia & Bardalai 2018). Biomass with higher volatile matter is considered suitable for pyrolysis due to high reactivity and better de-volatilization that lead to the release of gases such as methane, hydrogen, carbon monoxide, and carbon dioxide (Saikia & Bardalai 2018).

In TPD analysis of SCSW as shown in Figure 1(c), the single desorption peak between 600 and 700 °C represents the strong acidic sites due to acidic functional groups (OH, C = O, or COOH) (Gadelha et al. 2023). The concentration of acidic sites is found to be 96 μmol/g which is comparable with the concentration of an aluminum hydroxide fluoride catalyst (94 μmol/g) (Hemmann et al. 2014). Since these acidic groups are of high strength, this could facilitate the adsorption of basic dyes onto the surface of SCSW. Further adsorption of basic dye can be explained through pH_pzc analysis of SCSW, as shown in Figure 1(d). It depicts that the pH_pzc of SCSW is 6.2, which indicates that it has a negatively charged surface above pH 6.2 and positively charged surface below 6.2. Hence, it can be perceived that SCSW will attract positive ions in basic pH range and negative ions in acidic pH range.

Biochar has shown encouraging outcomes in the process of removing pollutants from wastewater due to its increased carbon content, surface area, morphological structure, and surface charge (Wakejo et al. 2023). To examine the same, ultimate, proximate, BET surface area, SEM, XRF, and point-of-zero charge analysis of ABC are performed and results are tabulated in Table 1. In ultimate analysis of ABC, given in Table 1, H/C and O/C ratios are found to be 0.57 and 0.4, respectively, which lie within the range of char according to the Van Krevelen diagram. Proximate analysis shows that ash and fixed carbon content are increased whereas volatile matter and moisture content are decreased in the carbonization process from SCSW to ABC. It is a result of progressive concentration of minerals and destructive volatilization of ligno-cellulosic matters under high temperature (∼500 °C) result. According to XRF results of ABC, given in Table 1, the most prevalent constituents are CaO (29%), SiO₂ (14.6%), and Na₂O (12.1%). Other oxides such as iron, magnesium, and aluminum oxides are found in a significant amount as well, with percentages of 5.3, 4.2, and 3.5%, respectively. Table 1 lists the BET surface area, pore volume, and pore diameter of ABC as 300 m²/g, 0.237 cm³/g, and 31.67 Å, respectively. It shows that porosity and surface area are greatly enhanced after thermal treatment due to devolatilization at higher pyrolysis temperatures. This leads to an increase in surface area as well as enhanced porous morphology. Both surface area and pore volume are improved by ∼115 times compared to SCSW. The adsorption–desorption isotherms of SCSW, given in Figure 1, shows a linear increase in volume adsorbed with the increase in relative pressure that exhibits the type II isotherm where unrestricted mono-multilayer adsorption takes place. Pore size distribution plot of ABC confirms the presence of mainly micropores (≤20 Å) as well as mesopores (20–200 Å) (Das & Debnath 2021).

FE-SEM images of ABC, given in Figures 2(b) and 2(c), represent the honey-comb structure similar to SCSW; however, wider and deeper pores with enhanced porosity and roughness are visible in ABC due to the release of volatile organic matter during the pyrolysis process which is confirmed by BET surface area analysis also. When surface roughness is increased, the surface tends to become more hydrophilic, leading to a reduction in the contact angle between the surface and the interface that typically falls below 90° (Li et al. 2021). Another significant factor is the presence of charged groups on the surface of the adsorbing medium which contribute to increased hydrophilicity at the interface between the adsorbate solution and the adsorbent surface (Li et al. 2022). In the present work, the NH₃-TPD analysis of SCSW indicates a substantial presence of strong acid sites on the surface that correlates with the existence of strong charged groups. Consequently, the MB dye solution exhibits hydrophilic tendencies toward the surface of the SCSW.

Figure 7(b) depicts the influence of ABC doses on the dye removal and adsorption capacity by varying dose from 2 to 15 g/L while temperature, pH, and dye concentration are fixed at 30 °C, 6.5 (natural pH), and 20 ppm, respectively. It is visible that the dye removal is enhanced with the ABC till 5 g/L and then becomes constant till 15 g/L. Hence, the optimal dye removal obtained at 5 g/L is 97% and further increase in ABC mass does not affect the removal due to unavailability of enough MB binding sites since dye concentration is constant (Wakejo et al. 2023). On the other hand, the adsorption capacity of ABC is decreased continuously with the rise in ABC dose from 2 to 15 g/L and higher adsorption capacity is found at the lowest adsorbent dose, i.e. 2 g/L due to an increase in the ratio of dye to adsorbent molecules (Zubair et al. 2020). When adsorbent dose is increased keeping dye concentration constant, the availability of binding sites per dye molecule increases which results in lower adsorption capacity (Zubair et al. 2020). It indicates that most of the adsorbent sites are not participating in adsorption and remain idle in the solution which can be used if the dye concentration is increased.

The initial dye concentration is likely one of the most significant parameters affecting the adsorption process since it directly affects the dye removal and adsorption capacity as mentioned in Equations (3) and (4). The influence of dye concentration on the adsorption of MB is studied by varying the concentration from 20 to 200 ppm and is shown in Figure 6(c). Other parameters such as pH, ABC dose, and temperature are kept fixed at 6.5 (natural pH), 5 g/L, and 30 °C. The dye removal is maximum at low dye concentration, i.e. 20 ppm and keeps on decreasing with increasing dye concentration. Since the ABC dose is constant for all the variation in dye concentration, the number of dye molecules available to bind with the active sites of ABC increases with the increase in dye concentration and compete for active sites (Wakejo et al. 2023). Hence it can be perceived that at 20 ppm, enough dye molecules are available to fully bind to the active sites present at 5 g/L of ABC dose and results in highest dye removal as shown in Figure 7(b). In contrast to dye removal, adsorption capacity is increased with initial dye concentration till 160 ppm due to a decrease in mass transfer resistance (Wakejo et al. 2023); however, further increase in dye concentration to 200 ppm decreases the adsorption. This could be due to the rise in equilibrium adsorption capacity with increasing initial dye concentration (Wakejo et al. 2023).

The temperature at which the adsorption reaction occurs is a crucial parameter, as it can shift the nature of the reaction from endothermic to exothermic and vice versa, thereby affecting the reaction's outcome. To study the influence of reaction temperature on the adsorption of MB on the ABC surface, experiments are performed by varying temperatures from 30 to 50 °C and keeping pH, ABC dose, and dye concentration fixed at 6.5, 5 g/L, and 20 ppm, respectively. Figure 7(d) shows that dye removal as well as adsorption capacity increases with temperature due to enhancement in reaction rate and maximum value is attained at 50 °C (Rápó & Tonk 2021). It also indicates that the adsorption of MB onto the ABC surface is an endothermic adsorption because the activation of the adsorbent surface takes place at higher temperature, facilitating the mobility of large dye ions toward the active sites (Rápó & Tonk 2021).

Based on parametric studies, the optimal conditions to maximize the MB dye removal and adsorption capacity are selected and experimental runs are performed. The optimal conditions for dye removal and adsorption capacity are estimated as 10 pH, 10 g/L of the ABC dose, 20 ppm and 50 °C and 10 pH, 2 g/L of the ABC dose, 160 ppm, and 50 °C, respectively, which results in 99.6% dye removal and 66.5 mg/g of adsorption capacity. Therefore, it can be inferred that biochar derived from SCSW possesses outstanding adsorbent properties, making it a viable option for efficient implementation in wastewater treatment processes. A comparison study, as illustrated in Table 3, reveals that ABC demonstrates superior adsorption capabilities compared to alternate biosorbents for the adsorption of the MB dye. However, in case of pollutants other than MB, MCM-48 and Co/MCM-41 exhibit the highest adsorption capacity for 4-nitroaniline and sulfur, respectively, using a fixed bed adsorption column.

This study investigates the strategies to utilize SCSW from the essential oil industry for two purposes: bio-oil production and the treatment of wastewater through adsorption. TGA and FTIR study reveals the presence of biopolymer functional groups (cellulose, hemicellulose, lignin), suggesting its suitability for bio-oil production. Additionally, TPD and pH_zpc analysis confirm strong acidic surface sites on the SCSW sample, ideal for adsorbing basic dyes. In light of the aforementioned details, the pyrolysis of SCSW at 500 °C results in the production of a significant quantity of bio-oil (45 wt%) and biochar (25wt%). Bio-oil comprises of carboxylic acids, phenols, aromatic hydrocarbons, and aliphatic hydrocarbons, whereas biochar (ABC) exhibits a significant augmentation in surface area and roughness, presenting robust adsorption sites in comparison to SCSW. Upon comparing the efficacy of ABC and SCSW for MB dye adsorption, ABC emerges as the superior adsorbent, achieving a maximum dye removal of 99.6% and an adsorption capacity of 66.5 mg/g under optimal conditions. These conditions include 10 pH, 10 g/L of the dose, 20 ppm of dye concentration, and 50 °C for dye removal and 10 pH, 2 g/L of the ABC dose, 60 ppm of dye concentration, and 50 °C for adsorption capacity. Kinetic study reveals that SCSW and ABC both follow pseudo-first-order kinetic. Hence, it can be concluded that SCSW not only demonstrates remarkable potential in wastewater treatment as an adsorbent but also emerges as an exceptional feedstock for bio-oil production. This two-fold utilization of SCSW represents a sustainable and circular approach, harnessing its full potential and contributing to a greener, more resource-efficient future.

The authors would like to thank Shiv Nadar (Institution of Eminence Deemed to be University) for providing experimental and financial support. The authors would also like to thank Head of the Department, Department of Chemical Engineering, IIT Roorkee for the characterizations (BET, NH₃-TPD).

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.