A Novel Preparation and Application of Orange Peels Aerogel for Removal of Oil Contaminants in Soils

1. Introduction

Global population is projected to hit 9.8 billion by 2050. As the population grows, the demand for land resources to develop infrastructure, such as housing, and to accommodate human activities increases. This expansion leads to higher extraction of natural resources, greater waste production, and intensified pressure on limited land resources. With increased mining to meet growing needs (Michaux 2021), weak legislation and lack of compliance (Sam et al., 2023), land resources continue to be contaminated. However, existing remediation approaches are time consuming and seldom inefficiently in restoring contaminated sites to meet societal needs and demands (Boulkhessaim et al., 2022; Lee et al., 2024). There is also an increased need to integrate a circular economy principle to addressing land contamination to reduce household waste.

Although different remediation methods exist. there are critical limitations. For example, while chemical methods are effective and could engender rapid remediation of hazardous waste, their impact on the natural environment and resource utilization could affect the achievement of sustainable development goals (Vlet et al., 2017; Smith et al., 2019), and its general societal acceptability. Evidence from research indicates that bioremediation could be the most sustainable approach for removing certain hazardous waste (Lee et al., 2023), especially as they are reported to account for almost zero greenhouse emissions, however, they are time consuming, and often unable to biodegrade recalcitrant contaminants (Kästner et al., 2016). Thus, there is an increasing need for rapid, efficient and sustainable remediation approaches (Azuazu et al., 2023).

Aerogels are highly porous three-dimensional materials made from organic and inorganic substances with low density and high specific surface area that can easily be adjusted (Zhu et al., 2017). They are a fascinating type of adsorbents having superhydrophobic and super oleophilic qualities and have demonstrated high sorption capacities (Keshavarz et al., 2021). Studies have shown that aerogels prepared from organic and inorganic materials have been applied in different areas, such as adsorption of contaminants including dyes, heavy metals, and oil from water and air (Zhu et al., 2017; Keshavarz et al., 2021). Aerogels produced from organic sources such as waste biomass are abundant and renewable, reduces dependence on fossil-based chemicals for aerogels production, promoting circular economy practices, lowering greenhouse gas emissions, and encourages the advancement of solvent-free or low-toxicity aerogels systems (Sheesley, et al., 2003; Kelly et al., 2018). The preparation method, structural properties, and performance of cellulose aerogels will largely depend on the biomass type and the amount of cellulose in the biomass. Aerogels prepared from agricultural wastes has received wide application in environmental remediation, due to the amount of cellulose in agricultural wastes (Li et al., 2020; Wang et al., 2020). The fabrication of aerogels from agricultural wastes is inspired by circular economic principles, and the need to reduce amount of waste to landfill (Franco et al., 2021). Also, because most agricultural wastes are cellulose based and abundant in nature, they are cheap materials for fabricating aerogels. Considering that they are soluble in water and organic solvents, this makes them simple and easy to undergo chemical modifications (Tu et al., 2022; Cai et al., 2017; Singh et al., 2017).

Cellulose is high in hydroxyl groups. This justifies the reason most agricultural wastes aerogels do not require crosslinking agent during the preparation phase (Tran et al., 2020; Dai et al., 2023). They are soluble in water and organic solvents making them easier to modify (Long et al., 2018). The drying process is the most critical stage in aerogel preparation. There are two different methods of drying cellulose-based gels, the supercritical fluid method, and the freeze-drying method. The supercritical method usually shows a cauliflower-like structure, while the freeze-drying method shows sheet-like cellulose structure with large, interconnected pores (Franco et al., 2021). Freeze-drying is simple, economical, and an environmentally low risk drying process compared to the other methods and can be used to create a highly spongy aerogel with minimal carbon footprints (Do et al., 2020; Dilamian & Noroozi, 2021; Jumbo et al., 2022). During freeze drying the liquid within the sample is solidified and then transferred under very low pressure by a vacuum pump, making the material spongier than other aerogels with different drying methods (Dai et al., 2022; Dilamian & Noroozi, 2021). The freezing intensity plays key role in the freeze-drying method and can change the consistency of pores (Verma et al., 2020).

According to reports cellulose-based aerogels prepared by the freeze-drying method are among the most promising and studied aerogels for adsorption of contaminants (Nguyen et al., 2019; Wang et al., 2020; Pung et al., 2022). Zhu et al. (2017) stated that aerogels derived from freeze-drying and carbonization processes showed good interconnected mesoporous structure, large specific area of 734.96 m²/g needed for adsorption. Also, good structural morphology and adsorption capacities was derived from pineapple fibre and banana stem (Jeng et al., 2020; Tu et al., 2023). The summary of existing studies on agricultural wastes aerogels in Table 1 shows that significant number of agricultural wastes have been successfully modified as cellulose aerogels. However, the results obtained from using waste biomass based cellulose aerogels particularly in contaminants remediation is variable due to factors such as hydrophilicity and need chemical modification, selective absorption of oil, and poor structural integrity especially in harsh or industrially contaminated environments (Muhammad et al., 2024; Ouyang et al., 2023; Table 1).

There is scarce literature evidence on studies of aerogels that can sustainably remediate contaminated soils due to these limitations. However, the elemental composition of orange peel indicates a promising feature in aerogel for sustainable contaminants remediation in soils (Table 2). As indicated in Table 2, adsorption capacities of orange peels can be significantly improved by modifying their structural properties (Michael-Igolima et al. 2023). In this context, superhydrophobic and oleophilic aerogels derived from orange peels using the freeze-drying method were innovatively applied in a modified adsorption technique to investigate its remediation potency in oil contaminated soils.

2. Materials and Method

2.1. Preparation of Superhydrophobic and Oleophilic Orange Peels Adsorbent

2.1.1. Physical Modification Process

Superhydrophobic aerogels were prepared by combined methods of physical and chemical modification processes. Fresh orange peels sourced locally from a smoothie shop in Bristol, England, were washed in fresh tap water to remove impurities and dried in oven at 75^o C for 48hours. The dried peels were ground and sieved to powder using a 300µm sieve. The obtained orange peels powder was used in the chemical modification process.

2.1.2. Chemical Modification Process

According to studies, incorporating relevant chemicals into a biomass improves the structural properties of the biomass and increases the adsorption capacities (Feng et al. 2009). Treating orange peels with a base such as NaOH(aq) increases the number of carboxylates and binding sites on the peel by converting methyl esters to carboxylate groups (Feng et al. 2009; Liang et al. 2009). In this study, 150g of dried orange peel powder was soaked in a mixture of 500ml of (1 mol) NaOH(aq) and 500ml of ethanol in a fume cupboard at room temperature. After 24h, the mixture was decanted and washed with distilled water until PH7 was obtained using a PH meter. Then the sample was dried for 48h in an oven at 75^oC to obtain the chemical modified orange peels (CMOP).

2.1.3. Thermal Modification Process

Research has shown that chemical treatment increases the number of binding sites on the biomass (Feng et al., 2009), therefore heating the biomass at high temperatures will increase the adsorption capacities of a biomass (El-gheriany et al. 2020). Research has shown that calcination of orange peels at temperature range of 400^o C to 500^o C increase adsorption capacities of the peel, however, the rate of adsorption starts to decrease at temperatures above 500^o C (El-Gheriany et al. 2020). The thermal modified orange peels were obtained by carbonizing the chemical modified orange peels (CMOP) in a furnace at 500^o C for 1hr as previously reported by El-Gheriany et al. (2020).

2.1.4. Preparation of Aerogels

To prepare aerogels, 5g of modified orange peels were added to 150ml of water in a glass beaker, and 3g of Oxoid CM0003 Nutrient Agar powder was also added to the beaker. The beaker was placed on a hot plate and stirred continuously with a magnetic stirrer, after about 15minutes the mixture was poured into a plastic beaker and left to cool to obtain an orange peels gel. Drying of the orange peel gel being the most critical step in aerogel preparation was carried out using the freeze-drying technique. For the aerogels to form, the samples were frozen at 0^oc for 24hours, then freeze dried for 48hours using a freeze-drying machine and an aerogel is formed. A pictorial summary of different stages involved in the preparation of superhydrophobic, and oleophilic orange peels aerogel is shown in Figure 1.

2.2. Characterization of Prepared Aerogel

The prepared aerogel was characterized using FEI Quanta 650 field emission scanning electron microscope and Fourier transform infrared spectroscopy (FT-IR) to study the morphology, and chemical properties of the aerogel. In addition, surface area analysis of the sample was carried out using Keyence digital microscope.

2.2.1. Scanning Electron Microscope

The structural morphology of fabricated aerogels was studied with FEI Quanta 650 field emission scanning electron microscope. The samples were coated with gold colored foil to increase their conductivity and mounted with conductive double-coated carbon tape adhesive. The obtained SEM micrographs of prepared aerogel was studied and was compared with images of other cellulose based aerogels found in literatures.

2.2.2. Fourier Transform Infrared Spectroscopy (FT-IR)

FT-IR spectroscopy of the aerogels was conducted with FT-IR spectrometer in the range of 400 to 4000cm^-1 to determine the chemical composition of prepared aerogel.

2.2.3. Surface Area Analysis

The pore diameter, and the average pore size of prepared aerogels was determined using Keyence digital microscope. Bulk density was calculated using the equation.

$$\mathrm{Bulk} \mathrm{density}\mathrm{d}=\frac{\mathrm{m}}{\mathrm{v}}$$

Eq1

where M is the mass of aerogel measured by weighing the aerogel, V is volume of the aerogel calculated by measuring diameter, thickness, and weight of prepared aerogels.

Porosity was determined manually using the equation 2, as shown in Trans et al. (2020).

$$\mathrm{Porosity} (\%)=1-\frac{\mathrm{d}}{\mathrm{p}}$$

Eq2

where d is bulk density of the sample and p is true density of crystalline cellulose (1.528g/cm³)

2.3. Hydrophobic Test

2.3.1. The Floating Test

The hydrophobic test of orange peels aerogel was conducted by visual examination using floating test. This involves dipping the aerogel in a transparent glass beaker containing water to observe the extent of floatation exhibited in water.

Water Contact Angle Measurement

The water contact angle of prepared aerogel was measured using a water contact angle meter (goniometer), three measurements were taken for each sorbent at separate locations on the aerogel to obtain the mean water contact angle of the aerogels.

2.4. Adsorption Experiments

Adsorption experiment was conducted in both sandy and clay soil to determine oil adsorption capacities of prepared aerogels in the media. Unconsolidated homogenous mixture of sand and clay soils was packed separately into 2000ml plastic beakers and allowed to consolidate for two weeks. Holes were drilled into the consolidated soil samples up to the bottom of the soil container (both for sand and clay samples), wide enough for the aerogels to enter the hole. 200ml of sunflower oil was poured into a glass beaker, as shown in Table 3. The adsorbents were fixed into the drilled holes, before uniformly spilling the oil on the soil and a contact time of 60minutes was allowed following the ASTM F 726-99 standard method for testing oil spill adsorbents (Figure 2). The aerogel was initially weighed to determine the initial weight before it was put in a perforated paper napkin and placed in open holes created in the soil. The napkin is to prevent the attachment of soil to the aerogel, which may alter oil adsorption result of the Orange peels aerogel (Figure 2). A contact time of 60minutes in the soils (sandy, clay) was allowed for the orange peels aerogel to adsorb more oil. The aerogel was removed from soil, weighed, and the new weight of the aerogel was recorded. The process was repeated in triplicates and recorded in Table 3. The absorbed oil was removed from the aerogel using the squeezing method as reported in Ifelebuegu et al. (2016). Ability of the adsorbent to extract oil was evaluated by investigating the rate of removal, and the adsorption capacities of the aerogels (mg/g) and was obtained using the equation in Imran et al. (2020), as shown in equation 3.

$$\mathrm{Q}=\frac{\mathrm{WF}-\mathrm{WI}}{\mathrm{WI}}$$

Eq3

where Q is the sorption capacity of adsorbent, Wi (g) is initial weight of the adsorbent, and WF is the weight of the adsorbent after adsorption.

To ensure the accuracy, reliability, and reproducibility of the collected data, all biosorption experiments were performed in triplicates.

3. Results and Discussion

3.1. Morphology and Structure of the Aerogel

Aerogels are characterised by an open, highly porous, low density, and air-filled structure with high degree porosity and surface area (Anovitz & Cole 2015; Franco et al., 2021). They have very high adsorption capacities due to their peculiar properties (Nita et al., 2020; Franco et al., 2021).

3.1.1. Densities and Porosities

The results of characterisation of the orange peels aerogel were shown in Table 4. From Table 4, the surfaces area has a mean pore size of 58nm, the diameter 51nm, and porosity of 99%. In the table, the aerogel had ultra-light density of 0.010417g/cm³, which makes it a good material for adsorption of contaminants. These obtained results (Table 4) corroborate with the research of Luu et al. (2020), Phat et al. (2022), and Cai et al. (2017) where the researchers independently used pineapple leaf, coconut peat, and cabbage-based aerogels for adsorption of oil and concluded that organic aerogels are good oil adsorbents. Table 5 showed absorption properties of aerogel produced from various agricultural biomass waste, however, compared with the results from Table 4, the produced orange peel aerogel has improved properties and more suitable for adsorption of oil from soil having shown higher porosity of 99%, and a minimum adsorption capacity of about 5.3 g/g.

3.1.2. SEM Image of Prepared Orange Peel Aerogel

Scanning electron microscope (SEM) was used to study morphological structure of prepared orange peels aerogels. The image in Figure 3 is a SEM micrograph of orange peels aerogel showing a rough and porous structure. The image in Figure 3 has an average pore size of 51nm (Table 4), which implied a high specific surface area for enhanced adsorption (Shi et al. 2019; Pung et al. 2022). Figure 3 has a porous structure that is loosely packed with open spaces which enhanced the adsorption of oil. Similar image has been reported for grapefruits aerogel though with reduced porosity (Imran et al., 2020). High adsorption capacities of aerogels can be attributed to the pore size and density of the aerogel which subsequently enhances adsorption of oil and organic solvents (Huang et al. 2022; Dai et al. 2022).

3.1.3. FT-IR of Prepared Orange Peel Aerogel

FT-IR spectroscopy was used to analyse chemical composition of prepared orange peel aerogels (Figure 4). The characteristic peak at 3374.93cm^-1 which stretches to 3400cm^-1 indicated the stretching vibration of hydroxyl groups in orange peels. This was reduced to 3278.41cm^-1 in the aerogel sample (Figure 4), which could be linked to the destruction of the hydroxyl functional groups during the freeze-drying process. This finding is corroborated by Do et al. (2020) and Thai et al. (2020) where it was stated that prolonged drying negatively impacts on hydroxyl functional groups during the formation of aerogel. The peak at 2917.86cm^-1 shown by the stretching vibration of C-H groups was increased to 2922.65cm^-1 (Figure 4). The bands in 1732.44cm^-1 and 1644.52cm^-1 assigned to the carboxyl and ester groups respectively are present at 1614.85cm^- and 1409.85cm^-1 in the aerogel sample (Figure 4), is attributed to the removal of majority of lignin and hemicellulose after treatment with NaOH. This corroborates reports of Nguyen et al. (2022) and Tu et al. (2022) that pretreatment process by alkalization effectively removes lignin and increases the cellulose.

3.1.4. Hydrophobic Tests of the Aerogel

Water droplets on the sample shown in Figure 5 formed a spherical shape on the surface which was used in measuring water contact angle values of the aerogel (Figure 5). Sessile image of the sample in Figure 5 (a) reveals that water droplets could stand and maintain a spherical shape on the aerogel surface for more than 30secs, showing an average water contact angle of 102.7^oC. The water contact angle of a good aerogel should range from 100^oC to 160^o C, and this range of angle influences the rate of adsorption in cellulose materials (Pan et al. 2011; Yue et al. 2018). Also, Yue et al. (2018) and Do et al. (2020) in their research concluded that aerogels with water contact angles higher than 100^oC have high adsorption capacities. Thus, the water contact angle value of 102.7^oC reported by this study (Figure 5) showed that the orange peels aerogel is a superhydrophobic and oleophilic material. To confirm the hydrophobicity a visual examination using the floating test in water was performed with result shown in Figure 5(b). The aerogel was observed to float in water (Figure 5(b)) with a uniform mirror reflection observed on the surface of the sample which may be caused by formation of interface between entrapped air in the aerogel and the surrounding water due to the surface structure shown in Figure 3 and the presence of the hydroxyl group shown in Figure 4. This corroborates the report of Yue et al. (2018) on the surface wettability of banana peels aerogels and wastepaper aerogels where the researchers stated that wettability can be related to presence of the hydroxyl group.

3.2. Oil Adsorption of Prepared Aerogels

Results of the oil adsorption test (Table 6) shows that sunflower oil penetrated the orange peels aerogel upon contact with the aerogel and have oil adsorption capacities of 13.55 mg/g in the sand and 9.60mg/g in clay soil. The high oil adsorption capacity of the orange peel aerogel in both soil (sandy and clay) was attributed to the ultra-light density of 0.010417g/cm³ and high porosity of 99% of the aerogel (Table 4). The investigation further showed that oil adsorption capacity of the orange peels aerogel was more in sandy than in clay soil, which was attributed to the soils texture and properties. This research agreed with the research of Atai et al. (2023) where the researchers stated that remediation of oil vary with soil types.

The oil adsorption capacities of three orange peels adsorbent materials, including pristine orange peels adsorbent, chemical modified orange peels adsorbent, and orange peels aerogel adsorbent in Table 6 showed 60% oil adsorption capacity for the orange peels aerogel, and a 20% oil adsorption capacity for chemical modified orange peels, while the pristine orange peels adsorbent had very low adsorption capacity lower than 1%. The result shows that the orange peels aerogel has the highest oil adsorption capacity and more suited for the remediation of oil in sandy and clay soils. This finding was corroborated by Lv et al. (2023) where the researchers stated that aerogels can accelerate remediation of oil polluted soils.

4. Implications of the Study

As transition from fossil fuel dependency to renewable energy sources accelerates globally, a substantial legacy of liabilities persists in many countries, notably numerous oil-contaminated sites left behind by decommissioned and non-decommissioned petroleum operations. These sites are often classified as abandoned and sometimes derelict due to the high costs and resource-intensive nature of conventional remediation methods (Sharma et al., 2018). Developing cost-effective, rapid remediation strategies holds promise for efficiently restoring these sites and facilitating their reuse, ultimately reducing environmental hazards and economic burdens.

This research demonstrates the potential of orange peel-derived aerogels as an innovative and sustainable approach for the remediation of oil-contaminated soils. Leveraging their intrinsic superhydrophobic and oleophilic properties, further enhanced through chemical and physical modifications, these aerogels exhibit high porosity and an extensive array of binding sites, rendering them highly effective for oil adsorption (Deschamps et al., 2003). The deployment of such materials in industrial and environmental remediation practices could introduce an innovative paradigm (Sam et al., 2023), enabling faster and more sustainable land clean-up processes. Moreover, when combined with existing bioremediation techniques, this approach could enhance remediation efficiency, thereby accelerating the reclamation of contaminated sites in an environmentally responsible manner.

The intrinsic biodegradability of the orange peel aerogel is a critical feature derived from its high cellulose content. As a naturally occurring biopolymer, cellulose exhibits excellent biocompatibility and undergoes biodegradation through microbial activity in the environment. This characteristic ensures that the aerogel materials can be used repeatedly over multiple remediation cycles without accumulating persistent environmental residues. Additionally, at the end of their functional lifespan, these aerogels can be safely degraded in situ, minimizing long-term ecological impacts and aligning with environmental sustainability objectives. Consequently, orange peel aerogels present a sustainable solution not only for effective remediation but also for eco-friendly disposal, reinforcing their role as a circular and environmentally responsible technology.

An additional advantage of this approach is its resourcefulness in reducing orange waste, aligning with circular economy principles aimed at zero waste generation. Transforming orange peels, an abundant agricultural by-product, into functional remediation materials offers a sustainable waste management solution and also significantly reduces volume of waste moved to landfill. Consequently, this strategy could mitigate leachate production from landfills, a major pathway for groundwater contamination, thus contributing to the protection and preservation of critical water resources.

5. Conclusions

A superhydrophobic and oleophilic aerogel was successfully produced with orange peels wastes. The produced aerogel had improved surface roughness and porosity alongside increased binding sites by converting methyl esters to carboxyl groups. SEM micrograph of the orange peel aerogel showed a rough and porous structures with high specific surface area of mean pore size of 51nm. The porous structure was loosely packed having open spaces which enhanced oil adsorption of the aerogel thereby increasing its oil remediation capacity. The freeze-drying process reduced the stretching vibration of the hydroxyl groups in the aerogel by 3.6%, therefor in producing aerogel using orange peels waste, there is need for controlled freeze drying to achieve optimum output. Similarly, reduction in the bands of 6.8% and 14.3% for carboxyl and ester groups respectively was found in the aerogel sample which is linked to the removal of majority of lignin and hemicellulose after treatment with NaOH. The sessile image of orange peels aerogel revealed that water droplets could stand and maintain a spherical shape on the aerogel surface for more than 30secs, with an average water contact angle of 102.7^oC. A further hydrophobic test conducted by visual examination using the floating test, revealed that the aerogel remained afloat in water indicating that the orange peels aerogel is a superhydrophobic and oleophilic adsorbent. The oil adsorption capacities of the prepared orange peels aerogel were 13.55 mg/g and 9.60 mg/g respectively for sandy and clay soils. The high oil adsorption capacity is attributed to the ultra-light density of 0.010417g/cm³ and high porosity of 99% of the aerogel. The study concluded that the higher oil adsorption capacity by the orange peels aerogel in sandy soil than clay soil indicated that soil texture and aerogel properties influence the oil remediation capacity of orange peel aerogels. In sum, this study promotes resourcefulness, and reduce reliance on traditional less sustainable remediation methods. The outputs of this study can be adopted in rapid remediation of oil polluted fields and wetlands.