Unlocking the potential of walnut husk extract in the production of waste cooking oil-based biodiesel

Highlights

• Solar photovoltaic-driven walnut husk extract was used as biodiesel antioxidant.

• The health aspects of waste-oriented biodiesel were improved.

• A potentially carcinogenic antioxidant was replaced with an agro-waste counterpart.

• Antioxidant capacity of natural antioxidant was comparable with that of propyl gallate.

• LCA results found the bio-antioxidant are more advantageous in all impact categories.

Abstract

Biodiesel has a lower oxidation stability index (OSI) than mineral diesel fuel. Its consequential oxidation products and deteriorated physical and chemical properties of fuel are associated with engine operation challenges such as the formation of insoluble gums that can plug fuel filters. Given the fact that oxidation leads to barriers for commercial use of biodiesel, addition of appropriate antioxidants into biodiesel is a promising and cost-effective approach to overcome this challenge. Although synthetic antioxidants such as propyl gallate (PG) are frequently used to counter the oxidation process of biodiesel, PG is a designated carcinogen. In light of that, this study was conducted aiming at introducing walnut husk methanolic extract (WHME) as a more sustainable antioxidant to replace PG in waste cooking oil (WCO) methyl esters. Moreover, to facilitate the commercialization of the new product, a comprehensive environmental investigation and comparison with the conventional counterpart, i.e., PG, was performed using life cycle assessment (LCA) approach. To enhance the eco-friendly features of the natural antioxidant, a solar photovoltaic-driven extraction process based on methanol (as reagent) was used in extracting polyphenols from walnut husk. The results showed that the induction period of WCO methyl esters was prolonged from 1.2 h to more than 3 h (meeting the ASTM D6751 standards) using 5000 ppm and 250 ppm of WHME and PG, respectively. More specifically, 20-fold more natural antioxidants would be required to meet the international standards. However, since walnut-producing countries are responsible for 42.4% of global biodiesel production on one hand and the cost-effectiveness of walnut husks on the other hand, their valorization could attract the attention of the global biodiesel industry. Moreover, this study highlights the considerable environmental and health benefits of turning this bio-waste product into a value-added antioxidant fuel additive. The LCA results showed that the developed bio-antioxidant was more effective in different damage categories compared with PG, i.e., 0.32% in ecosystem quality, 12.13% in human health, 8.37% in climate change, and 614% in resource. Overall, the WHME obtained through solar photovoltaic-driven extraction process could outcompete PG from the environmental perspective.

Introduction

One of the most significant current discussions in the world is the growing fossil fuel demands [1] and the concerns associated with the rapid depletion of these non-renewable energy carriers [2]. In addition, there exists increasing concerns about the consequent unfavorable environmental impacts caused by the resultant toxic emissions [3] as well as emissions contributing to climate change such as carbon dioxide (CO₂) [4]. These have spurred an intense search in both developed and developing countries for renewable energy carriers that could partially or completely replace fossil fuels [5].

Among these energy carriers, a blend of several fatty acid methyl or ethyl esters also known as biodiesel [6] has attracted a great deal of attention because of its biodegradable and nontoxic characteristics [7]. Biodiesel also promises to reduce problematic emissions such as particulate matter (PM), unburnt hydrocarbons (UHC), and carbon monoxide (CO) [8]. During the last decade, biodiesel has been recognized as a cost-effective and environmentally sound transportation fuel [9]. Biodiesel is primarily synthesized by transesterification of vegetable oils and animal fats with monohydric alcohols (ethanol-C₂H₅OH, methanol-CH₃OH) [10] using various basic/acidic catalysts (mostly potassium hydroxide-KOH or sodium hydroxide-NaOH) [11].

Currently, industrial production of biodiesel relies primarily on virgin oil feedstock [12] which has led to major challenges including high cost of feedstock as well as feedstock procurement [13]. Nevertheless, it has been well documented that the application of waste-product oil resources such as waste cooking oil (WCO) can significantly enhance the economics of biodiesel production [14] while resolving fuel supply issues in remote areas [15]. Meanwhile, WCO is discharged inappropriately into soils, water, and sewerage systems in many parts of the world [16]. As such, its reuse as biodiesel feedstock could lead to a reduction of WCO releases into nature.

In spite of its advantages, biodiesel’s relatively poor thermo-oxidative stability and short shelf life compared with mineral diesel due to its ester-based chemistry must be addressed to increase its viability as an alternative fuel [17]. Moreover, the oxidative stability of biodiesel depends on the physicochemical features of the oil feedstock used [18]. In comparison with virgin oil, WCO is generally far more sensitive to oxidative and thermal degradation [19]. Gums, aldehydes, alcohols, carboxylic acids, insoluble sediments, and other products of biodiesel oxidation can foul injectors and clog fuel filters and lead to the fusion of moving parts, failure of fuel systems, combustion chamber deposition, and tightening of rubber components [20]. Various approaches have been proposed to increase the oxidation stability index (OSI) (or the induction period (IP), i.e., the time period until the appearance of secondary reaction products [21]) of biodiesel, including storage in an inert nitrogen atmosphere [22], distillation [23], use of different raw materials [24], and application of antioxidants [25].

Among the mentioned approaches, special emphasis has been placed on the application of antioxidants to combat the free radicals generated in the initiation and propagation phase of fatty acid esters oxidation [26]. This is because antioxidants reduce the need for costly treatments as well as new storage tanks and fuel-handling systems [27]. Synthetic antioxidants, including tert-butyl hydroxyquinone (TBHQ) [28], N,N0-diphenyl-p-phenylenediamine (DPPD) [29], butylatedhydroxyanisol (BHA) [30], butylatedhydroxytoluene (BHT), propyl 3,4,5 trihydroxybenzoate (pyrogallol or PY) [31], propyl gallate (PG) [27], and other commercial products such as Baynox (Lanxess) [32], Ethanox 4760E (Albemarle) [33], and Bioextend (Eastman) [34], are frequently used to prevent oxidation in biodiesel and other fuels. All these antioxidants donate hydrogen from their hydroxyl (–OH) group(s) to free radicals, preventing progression of the oxidation process. As determined by the number of OH groups, these antioxidants are ranked as BHA, BHT < DTBHQ = TBHQ < PY = PG. Both PG and PY contain three OH groups linked to an aromatic ring [27]. While both PY and PG have more liable hydrogen, PY is less effective, probably owing to lower solubility in some biodiesels [27]. PG is now the most widely used antioxidant in the biodiesel industry. However, synthetic antioxidants, including PG, have been criticized because of reported adverse side effects. For instance, a National Toxicology Program study revealed a direct relation between PG and cancerous tumors in rats (US Department of Health and Human Services) [35]. Moreover, synthetic materials are often considered environmental pollutants as they are more complex compared with their natural counterparts, taking longer to biodegrade [35]. The search for alternatives to synthetic antioxidants has resulted in the investigation of numerous antioxidants found in plants such as olive leaf extract [36], rosemary (Rosmarinus officinalis L.), oregano (Origanum vulgare), and thyme (Thymus vulgaris) [37]. Unlike their synthetic counterparts, natural antioxidants are generally recognized as safe, and their applications are less limited [38].

Various studies have been performed to investigate the OSI of natural antioxidants vs. their synthetic counterparts. The OSI of soybean-oriented biodiesel supplemented with a number of alcoholic plant extracts such as rosemary, oregano, and basil has been assessed [39]. Soybean biodiesel samples containing antioxidants individually or in combination displayed higher resistance to oxidation as determined by Rancimat vs. control. A 1:4 mixture of rosemary and oregano extracts was found to be the most favorable treatment. The antioxidant effects of ginger extract on the stability of biodiesel produced from pongamia (a non-edible energy crop) have also been studied [40]. Accordingly, a minimum ginger extract concentration of 250 ppm was required to meet the European standard specifications (EN 14214) for biodiesel OSI. In a similar work, Devi et al. [41] evaluated potato peel extract in the range of 100‒250 ppm as an antioxidant for Mesua ferrea L. biodiesel. They argued that potato peel extract was more effective than the synthetic antioxidant TBHQ in enhancing the OSI of the investigated biodiesel. In a recent investigation, Ahanchi et al. [17] compared the effectiveness of pistachio hull extracts of different varieties in enhancing canola methyl ester OSI, with the synthetic antioxidant PG used as a reference. The results showed that pistachio hull extracts and PG concentrations of 2500 ppm and 250 ppm, respectively, were sufficient to prolong the IP of straight methyl esters from 1.53 h to over 3 h as per the American Society for Testing and Materials (ASTM) D6751 standards for biodiesel OSI. Despite the superior performance of the synthetic antioxidant, the authors concluded that pistachio hull extracts are a more favorable option from the human health point of view [10].

The environmental effects of newly identified natural antioxidants have also been scrutinized to further their sustainable production. Rodríguez-Meizoso et al. [42] introduced a novel method called water extraction and particle formation on-line to extract antioxidants from rosemary leaves. They used LCA approach to compare the environmental performance of their method with the other methods used to obtain antioxidants. They claimed that their method was associated with lower environmental impacts and energy consumption because it was carried out in a single step. They also highlighted the importance of the primary energy sources of the electricity used in the antioxidant production process [42]. In another study, the environmental aspects of astaxanthin production by microalgae were evaluated using LCA approach from lab to pilot scale [43]. The results identified electricity requirements as a major factor that affect the environmental impacts of the production process. It also claimed that the environmental impacts were greatly reduced in the pilot-scale production due to changes implemented following a lab-scale environmental assessment. Ahanchi et al. [17] used LCA approach to evaluate the environmental performance of antioxidant production from pistachio hulls and concluded that use of the produced natural antioxidant was promising from climate change and human health standpoints as compared with a synthetic antioxidant like PG. The authors stated that the implementation of a hexane recovery step (throughout the antioxidant production process) was the main reason for the reductions in the environmental impacts.

Overall, there are many methods to deal with agro-wastes in modern agriculture, however, their valorization into valuable biomaterials including natural antioxidants is regarded as a very promising strategy. This has led to a widespread search to identify agro-wastes containing significant amounts of phenolic compounds, which could be used for sustainable production of natural antioxidants. In line with that, antioxidant characteristics of different parts of the walnut plant (fruit, stem, leaf, and husk) have been investigated [44]. However, to the best of our knowledge, there is no reports published on the use of walnut husk methanolic extract (WHME) as a biodiesel fuel additive. Therefore, this study for the first time set out to explore the application of WHME as a natural antioxidant for WCO biodiesel. On the other hand, despite the high efficacy of solvent extraction, it suffers from high non-renewable energy consumption during solvent recovery which would undoubtedly be attributed to adverse environmental consequences. Therefore, a solar photovoltaic-driven extraction process was implemented in the present study to address this challenge. Following an investigation of the antioxidant properties of the extract, WHME was compared with the most commonly employed synthetic antioxidant, PG, using the Rancimat method based on the EN 14214 standard by measuring the OSI, as detailed in our previous study [17]. Finally, an LCA of the environmental burdens associated with the synthesis and utilization of the investigated antioxidant as a biodiesel fuel additive was performed. Considering the health risks associated with PG exposure and the fact that walnut husks (inexpensive and readily available agricultural waste products) could serve as an ideal antioxidant source, the outcomes of this survey could be of great interest to the biodiesel industry. In fact, the findings presented here could be of value in improving the health benefits of waste-oriented biodiesels around the world while also contributing to the integration of waste valorization and renewable energy implementation with waste-oriented biodiesel production.