001.docx DOI: 10.3303/CET2189080 Paper Received: 23 May 2021; Revised: 10 September 2021; Accepted: 3 November 2021 Please cite this article as: Ab Razak N.A., Mohd Hanafi M.H., Razak N.H., Ibrahim A., Omar A.A., 2021, The Effective Polycyclic Aromatic Hydrocarbons Removal from Waste Cooking Oils: The Best Evidence Review, Chemical Engineering Transactions, 89, 475-480 DOI:10.3303/CET2189080 CHEMICAL ENGINEERING TRANSACTIONS VOL. 89, 2021 A publication of The Italian Association of Chemical Engineering Online at www.cetjournal.it Guest Editors: Jeng Shiun Lim, Nor Alafiza Yunus, Jiří Jaromír Klemeš Copyright © 2021, AIDIC Servizi S.r.l. ISBN 978-88-95608-87-7; ISSN 2283-9216 The Effective Polycyclic Aromatic Hydrocarbons Removal from Waste Cooking Oils: The Best Evidence Review Nurul Afwanisa’ Ab Razak, Mohd Hafidzal Mohd Hanafi*, Nurul Hanim Razak, Asriana Ibrahim, Anis Ainaa Omar Centre for Advanced Research on Energy (CARe), Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia. hafidzal@utem.edu.my Polycyclic aromatic hydrocarbons (PAHs) are a large group of organic compounds that consist of two or more aromatic rings. PAHs are produced during incomplete combustions of hydrocarbons. Apart from being identified as carcinogenic and mutagenic, PAHs pose a high potential to cause severe health and environmental issues. Oils and lipid matrices, such as Waste Cooking Oils (WCOs), may contain highly concentrated amounts of PAHs that are unsafe. Although vegetable oils are mainly free from PAHs, they are exposed to PAHs contamination from environmental sources. To date, studies on the removal of PAHs in WCOs have been limited despite that many researchers have demonstrated the increasing health risk posed by WCOs. Therefore, this review aims to discuss the best-recommended method to treat WCOs, particularly for removing PAHs. A selected number of the most common biological and physical/chemical treatment methods for PAHs removal were reviewed. In short, this review concluded that the adsorption method using 1 % activated charcoal for 35 min at 110 °C under vacuum bleaching operation at 30 mbar to 35 mbar was considered the most approach to remove PAHs. The method successfully removed 99.7 % of benzo[a]pyrene, signifying the potential application of Activated Carbon (AC) to remove PAHs in WCOs. 1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are volatile compounds with a high propensity to adsorb onto organic matters and are insoluble in water (Paris et al., 2018). PAHs are produced during incomplete combustions of hydrocarbon due to non-ideal temperature, oxygen capacity, and high moisture. PAHs are started with small and larger hydrocarbons before soot is produced (Hanafi et al., 2018). In 1976, the United States Environmental Protection Agency (US EPA) released a list of 16 PAHs that were evaluated based on the risks towards human health, which later played a significant role as a standardised set of compounds to be analysed, especially in environmental studies (Andersson and Achten, 2015). The list of 16 PAHs include naphthalene, acenaphthene, acenaphthylene, anthracene, phenanthrene, fluorene, fluoranthene, benzo(a)anthracene, chrysene, phyrene, benzo(a)pyrene, benzo(b)fluoranthene, benzo(k)fluoranthene, dibenz(a,h)anthracene, benzo(g,h,i)perylene, and indeno[1,2,3-cd]pyrene. Oils and lipid matrices such as Waste Cooking Oils (WCOs) contain a highly concentrated PAH level that is harmful because of their lipophilic nature. Vegetable oils are generally PAHs-free, but PAHs could contaminate them through environmental sources, such as raw vegetable material and contamination during seed drying process. They could also be polluted through solvent extraction, soil burning, packaging material, mineral oil residues, and migration of contaminated water or soils (Teng et al., 2019). Edible oils are exposed to a considerable risk of PAHs contamination. The production of edible oils that involve the drying of vegetable seeds before the oil extraction process is susceptible to contamination from PAHs, which are contributed by the release of combustion gases rich in PAHs (Sánchez-Arévalo et al., 2020). One of the most prevalent reasons that the removal of PAHs should be heavily considered is the direct consumption of fats and oils in food. The volatile and nonvolatile compounds produced during chemical reactions the repeated usage of WCOs cause a carcinogenic effect on humans, such as increased tumor proliferation (Ganesan and Xu, 2020). Besides, several studies have shown that the consumption of WCOs with 475 saturated fat contents leads to high triglyceride levels in the blood that would subsequently affect the liver function of Wistar rats (Mulyati et al., 2020) and rabbits (Ambreen et al., 2020). A recent study has claimed that PAHs pose a high potential to cause health and environmental issues due to their carcinogenic and mutagenic effects (Razak et al., 2021a). It is vital to efficiently remove the hazardous emissions of PAHs contaminants (Razak et al., 2021b). Several researchers have focused on the efficient removal of organics pollutants from soil, water (aqueous solution), and atmosphere. The removal of PAHs is divided into biological and physical/chemical treatment methods, as visualized in Figure 1. Few works have attempted to remove PAHs in WCOs, while the available literature has not applied advanced treatment technologies with process parameters. To fill the gap, this review aimed to present the best-recommended method in WCOs treatment to remove PAHs. Figure 1: Variety of PAHs removal methods including biological and physical/chemical treatment process 2. PAHs removal treatment Various methods have been explored to reduce the potentially hazardous properties of PAHs towards the environment and human health. Some of the most common biological and physical/chemical treatment methods for PAHs removal are listed in Table 1. In this study, the literature review was accessed from the Scopus databases, and the data inquiry phrases were set as “Polycyclic Aromatic Hydrocarbons” AND “removal”. Based on the search scope, the references were refined to publication years (2012-2021) and type of language (English). A study conducted by Ma et al. (2017) proved that 99.7 % of benzo[a]pyrene was successfully removed with only 1 % activated charcoal at 110 °C for 35 min under vacuum bleaching operation at 30 mbar to 35 mbar. The advantage of this method was the minimal concentration of activated charcoal applied to yield an extremely high efficiency for benzo[a]pyrene removal. A similar finding was reported to removed 93 % of the total 11 PAHs through the utilisation of powdered SilCarbon TH90 AC with an average particle size of 20 µm (Hale et al., 2012). The high specific surface area of the AC indicates the superior performance of small powdered AC. In comparison, the main disadvantage of phytoremediation, rhizoremediation, and fungal remediation is the time consumption in which the approach took two years (Chen et al., 2016), 150 d (Liu et al., 2014) and three months (Winquist et al., 2014) to complete the removal process. Mohd-Kamil et al. (2014) reported that the utilisation of microbial treatment was limited to a contaminated site, which would require additional steps to enhance the performance of the bacteria for the ex-situ process. Researchers also concluded that the bioreactor process approach was constrained by complex operating procedures (Qiao et al., 2016), while the application of biosurfactants involves high investment and maintenance costs (Cameotra et al., 2010). 3. Mechanism of adsorption The adsorption mechanism of PAHs on AC is highlighted in Figure 2. Li et al. (2020) dictate that the mechanism is classified into weak and strong adsorption. The weak adsorption mechanism is mainly through hydrophobic interactions, Van der Waals forces, and hydrogen bonds. Hydrophobic interaction is determined by octane- water distribution coefficient (KOW) or the lipophilicity of a compound. Amino groups (-NH2), hydroxyl groups (- OH), and carboxyl groups (-COOH) in PAHs derivatives are readily adsorbed via hydrogen bond through the AC. The strong adsorption mechanism is due to 𝜋𝜋-𝜋𝜋 interaction, 𝜋𝜋 complexation, and electrostatic interaction. The aromatic rings in PAHs consist of a few 𝜋𝜋 electrons that can readily combine with aromatic rings or 𝜋𝜋 bonds on the AC. PAH Removal Treatment Methods Biological Process Bio- remediation Phyto- remediation Anaerobic Rhizo- remediation Microbial Fungal Bioreactor Bio-surfactant Physical / Chemical process Adsorption Cellulosic Aerogel Activated Carbon Clay Mineral Biochar Advanced oxidation 476 Table 1: Details of the process, removal rate, and key performance on the biological and physical/chemical treatment of PAHs An essential interaction between PAHs and AC is found in the 𝜋𝜋-𝜋𝜋 electron-donor-acceptance interaction. The 𝜋𝜋 complexation is formed by functional groups or heterocyclic molecules consisting of metal ions that can accept Reference Treatment methods Types of Process Process Removal rate Key Performance Chen et al. (2016) Biological Bioremediation Phytoremediation Ryegrass Seduce alfredii, Microbacterium sp. and Candida tropicalis, 2 y 96.4 % of the Σ 16 PAHs High efficiency, low cost and environmentally friendly Sherafatmand and Ng (2015) Anaerobic 76.9 % of naphthalene High level of pollution and limited oxygen flow Liu et al. (2014) Rhizoremediation Fire Phoenix, 150 d 99.4 % of the Σ 8 PAHs Increase the microbial diversity and promote the growth of major bacteria Mohd-Kamil et al. (2014) Microbial Sphingobacterium spiritivorum, 5 d 95.36 % of phenanthrene Great efficiency, natural and cost- effective Winquist et al. (2014) Fungal Phanerochaete velutina, 3 months 94 % of the Σ 16 PAHs High tolerant concentration of PAHs and extreme conditions Qiao et al. (2016) Bioreactor Aerobic activated sludge treatment, 7 d, 10-35 °C 83-97 % of 4 d PAHs Manipulate, monitor, and control environmental and operational variables Cameotra et al. (2010) Biosurfactants Fungal Paenibacillus dendritiformis, 5 d 96 % of phenanthrene, 83 % of pyrene Capable of lowering the surface and interfacial tension of the growth medium Kim et al. (2021) Physical/ Chemical Adsorption Cellulosic aerogel Eulalia, 150-240 °C, 10-30 min, 0.3-5.0 wt% 44.78 % of benzo[a]pyrene Reduce environmental pollution and enhance the application of by- product Ma et al. (2017) Activated carbon 1 % activated charcoal, 110 °C, 35 min, vacuum bleaching at 30-35 mbar 99.7 % of benzo[a]pyrene Excellent efficiency at very low content of AC addition Staninska et al. (2015) Modified mineral clay, humic acid, 24 h 99 % of phenanthrene Expanded three- dimensional (3D) structure of their molecular sieve properties Li et al. (2014) Biochar Wheat straw, 800 °C, 6 h 71.8 %-98.6 % of phenanthrene, fluoranthene, and pyrene High surface area, porous structure to adsorb contaminants, and high adsorptive capacity Ates and Argun (2018) Advanced oxidation processes Fenton, ozone oxidation, pH 3 >70 % of acenaphthylene, acenaphthene, phenanthrene, fluoranthene, and pyrene High energy consumption and high maintenance costs, low pH value 477 the delocalized 𝜋𝜋 electrons from the aromatic rings of PAHs. According to Dugyala et al. (2016), electrostatic adsorption is the opposite charge of the functional group of adsorbate and adsorbent. In this particular adsorption method, the adsorption involves AC with oil containing PAHs, where the mass transfer shifted towards the AC. The proposed adsorption mechanism of PAHs on AC is illustrated in Figure 3. When the PAHs in oil come into contact with a highly porous surface structure of AC, the liquid-solid intermolecular forces cause some of the PAHs molecules from the oil to be concentrated or deposited onto the AC surface. PAHs adsorbed on the AC surface during adsorption are called adsorbate, while the AC adsorbs PAHs are called adsorbent. Figure 2: The adsorption mechanism of PAHs onto activated carbon (Li et al., 2020) Figure 3: The proposed adsorption mechanism of PAHs on AC in oil 4. Factors affecting the rate of adsorption Generally, five significant factors affect the adsorption rate of PAHs on AC, including pH level, temperature, particle size and surface area, solubility, and contact time. 4.1 pH level According to Li et al. (2020), the ionic strength and pH level serve a crucial function in the adsorption and desorption of PAHs onto the AC. The bridging interaction of metal ions and the electron donor-acceptor interaction are the dominant mechanisms by which pH level and ionic strength are associated with the influence on the adsorption and desorption of PAHs. A study by Lamichhane et al. (2016) showed that the pH level of the solution affected the extent of adsorption capacity due to the distribution of surface charge of the adsorbent. The extent of adsorption was varied according to the functional groups of the adsorbate. An elevated positive charge on the adsorbent surface at low pH resulted in higher interaction between the adsorbent and PAHs molecules. The variation of pH level plays an important role in electrostatic adsorption for ionisable organic chemicals, such as PAHs. In brief, a higher PAHs removal rate was achieved using a solution with a lower pH level. 4.2 Temperature The effect of temperature on PAHs sorption could be described by Van't Hoff. Eq(1), which was derived using the concept of Gibbs free energy. ln 𝐾𝐾𝑒𝑒𝑒𝑒 = − ∆𝐻𝐻 𝑅𝑅 1 𝑇𝑇 + ∆𝑆𝑆 𝑇𝑇𝑅𝑅 (1) Adsorbent: AC Adsorbate: PAHs Liquid-solid Intermolecular force 478 where ΔH is change in enthalpy (kJ/mol), T is the absolute temperature (K), ΔS is change in entropy (kJ/mol/K), R is gas constant (8.314 x 10-3 kJ/K/mol), and Keq is equilibrium partition coefficient at a given temperature (cm3/g) (Lamichhane et al., 2016). In short, higher temperatures led to a greater PAHs removal rate under endothermic reaction. 4.3 Particle size and surface area Lamichhane et al. (2016) also found that the capacity of adsorbent PAHs adsorption was inversely proportional to the particle size of the media due to adsorption being a surface phenomenon. The extent of adsorption was proportional to the specific surface area, which refers to the portion of the total surface area available for adsorption. According to Li et al. (2020), the pore size distribution and specific surface area of AC significantly influenced PAH adsorption. In brief, a higher PAHs removal rate was obtained using adsorbents with smaller particle sizes and larger surface areas. 4.4 Solubility The solubility of the adsorbate greatly influences the adsorption capacity of the adsorbents. An inverse relationship was observed between the extent of adsorption of a solute and its solubility in the solvent. It was suggested that the solubility of PAHs exhibited an inverse relationship with molecular weight (Lamichhane et al., 2016). 4.5 Contact time When hydrophobic organic compound comes into contact with the media, a portion of the compound is rapidly adsorbed by the media components (such as organic matter, clay minerals) through physisorption. However, it takes longer for the remaining fractions to reach sorption equilibrium. The experimental result showed 70 % of PAHs removal was achieved within 2 h of contact time and then gradually reaching equilibrium within 24 h for maximum removal efficiency (Lamichhane et al., 2016). In summary, a longer contact time resulted in higher PAHs removal. 5. Conclusions In this review, the latest collection of research articles from the Scopus database relating to the highest removal rate of PAHs were presented. The effectiveness of the adsorption process depends on operating variables, including pH level, temperature, particle size and surface area, solubility, and contact time. Based on the recent studies, it was suggested that PAHs removal rate could be enhanced by maintaining a low pH, utilising adsorbents with smaller particle sizes and more significant surface areas, and extending the contact time. Among all the methods discussed, the adsorption using 1 % activated charcoal for 35 min at 110 °C under vacuum bleaching process at 30 mbar to 35 mbar was the most effective approach to remove PAHs. The method successfully removed 99.7 % of benzo[a]pyrene. Due to its excellent efficiency at very low content of AC addition, adsorption by AC is a possible promising method to be applied as WCOs treatment to remove PAHs significantly. Acknowledgments The authors are grateful to Universiti Teknikal Malaysia Melaka (UTeM) for the financial support through FRGS/1/2020/FKM-CARE/F00436. References Ambreen G., Siddiq A., Hussain K., 2020, Association of long-term consumption of repeatedly heated mix vegetable oils in different doses and hepatic toxicity through fat accumulation, Lipids in Health and Disease, 19(1), 1-9. Andersson J.T., Achten C., 2015, Time to say goodbye to the 16 EPA PAHs? 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