Abstract
Peels make up a considerable proportion of solid waste generated from fruit and vegetable production and processing. If not properly managed, they could contribute to environmental degradation through the dispersion of nutrient-rich leachate and the release of various greenhouse gases. Alternatively, these peels could be transformed to biosorbents, which could assist in the removal of pollutants of environmental and human health concerns from wastewaters. Using peels as raw material for biosorbent production is an environmentally friendly and cost-effective option for waste disposal. Peels also contain bio-activators, which can be used to activate the biosorbent produced, minimizing the use of synthetic chemicals for biosorbent activation. This review considers the different physicochemical characteristics of vegetable and fruit peels that make them suitable raw materials for biosorbent production. Additionally, their transformation to biosorbents using hydrothermal carbonization and pyrolysis is discussed. The review concludes with a discussion on the efficiency of peel-based biosorbents in the removal of diverse types of pollutants from wastewater.
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Fruit and Vegetable Peel Characteristics and their Conversion to Biosorbents using Hydrothermal Carbonization and Pyrolysis: A Review
Veronica M. Ngole-Jeme * and Christophe N. Ntumba
Peels make up a considerable proportion of solid waste generated from fruit and vegetable production and processing. If not properly managed, they could contribute to environmental degradation through the dispersion of nutrient-rich leachate and the release of various greenhouse gases. Alternatively, these peels could be transformed to biosorbents, which could assist in the removal of pollutants of environmental and human health concerns from wastewaters. Using peels as raw material for biosorbent production is an environmentally friendly and cost-effective option for waste disposal. Peels also contain bio-activators, which can be used to activate the biosorbent produced, minimizing the use of synthetic chemicals for biosorbent activation. This review considers the different physicochemical characteristics of vegetable and fruit peels that make them suitable raw materials for biosorbent production. Additionally, their transformation to biosorbents using hydrothermal carbonization and pyrolysis is discussed. The review concludes with a discussion on the efficiency of peel-based biosorbents in the removal of diverse types of pollutants from wastewater.
DOI: 10.15376/biores.19.4.Ngole-Jeme
Keywords: Hydrothermal carbonization; Low-cost adsorbent; Chemical activation; Pyrolysis; Bioactivators; Biochar
Contact information: Department of Environmental Sciences, College of Agriculture and Environmental Sciences, UNISA Science Campus Florida, Roodepoort, 1710, Gauteng, South Africa; Corresponding author: ngolevm@unisa.ac.za
GRAPHICAL ABSTRACT
INTRODUCTION
The complexity of chemicals contained in wastewater streams, coupled with poor management of industrial and domestic effluent discharges, have aggravated water pollution concerns globally. Both organic (dyes, phenols and benzene compounds, pesticides, fertilizers, hydrocarbons, detergents, oils, pharmaceutical, and personal care products) and inorganic pollutants (especially heavy metals) originating from untreated and partially and/or poorly treated wastewater are of environmental and human health concern because of their non-biodegradability, persistence, mutagenicity, toxicity, carcinogenicity, and teratogenicity (Bahadir et al. 2007; Georgieva et al. 2020; Kaur and Roy 2021; Goswami et al. 2022; Kumar and Kumar 2022). This concern is confounded by the fact that industries are increasingly making use of xenobiotics, which are not amenable to some of the available wastewater treatment methods. If these pollutants are allowed to reach water bodies, they could degrade their quality, aggravating the challenge of water availability globally, especially in developing countries. The control and removal of these pollutants from effluents prior to their discharge into water bodies is therefore a matter of urgency to ensure environmental sustainability.
Despite the availability of multiple physical (sedimentation, skimming, coagulation, electrochemical), chemical (precipitation, oxidation, reverse osmosis, ion exchange, sorption), and biological (aerobic/anaerobic degradation) wastewater treatment methods with a potential to remove both organic and inorganic pollutants from wastewaters, their complete elimination from wastewaters has not always been possible. The increased volumes of wastewater generated, low efficiency of treatment methods, and the high cost associated with some of these methods are responsible for this failure (Ambaye et al. 2021). One of the wastewater treatment methods most commonly used in industry is adsorption, which relies on the surface chemistry of a solid material (adsorbent) to physically or chemically bind either organic or inorganic pollutants (adsorbate) from a solution (Dotto and McKay 2020; Bilal et al. 2021; Chai et al. 2021; Praveen et al. 2021; Kumar and Kumar 2022). Adsorbents used in the removal of pollutants from water include silica gel, zeolite, alumina, activated carbon, and nanocomposites, among others (Mishra et al. 2022).
Silica gel is a highly porous form of silicon dioxide produced from quartz sand or any silica rich material through hydrolysis or condensation reactions (Kazemzadeh et al. 2012; Visser 2018; Azmiyawati et al. 2019). Zeolites on the other hand comprise mainly of a framework of tetrahedra that may contain either silicon or aluminum (SiO4 or AlO4), and in which each oxygen atoms at the four edges of the tetrahedra are shared with adjacent tetrahedra (Derbe et al. 2021; Britannica 2024). The linking of a silica tetrahedron and an alumina tetrahedron creates a charge imbalance that is usually neutralized by the presence of an alkali or alkali earth metal (Rehᾴkova et al. 2004). Zeolites may occur naturally, or they could be synthesized from glass materials, kaolin/metakaolin, coal fly ash, lithium slag, K-feldspar, and porcelain waste (He et al. 2016; Khaleque et al. 2020). The synthesis could be through alkali-fusion, sol gel, alkali leaching, or hydrothermal methods (Sugano et al. 2005; Wajima et al. 2008; Tsujiguchi et al. 2014; Shoppert et al. 2017). It is usually the synthesized zeolites, especially those synthesized from fly ash, that are used as adsorbents (Khaleque et al. 2020). Alumina, another commonly used adsorbent, is a whitish commercial adsorbent composed of aluminum oxide produced through either the Bayer process, sintering, hydrothermal synthesis, or sol-gel process (Banks and Bridgwater 2016). Alumina that is used as an adsorbent is usually activated (Rouquerol et al. 2014). Activated alumina is produced by dihydroxylation of aluminum hydroxide through calcination at temperatures of between 300 and 600 ℃ to create a highly porous structure with a high surface area (Banks and Bridgwater 2016). Activated carbon is one of the most popular adsorbents used in industry, and it is a highly porous, non-polar, amorphous material made from carbon rich materials such as bituminous and lignite, oil cake, and various biomaterials (Muttil et al. 2023). All these adsorbents are characterized by a high density of interconnected pores, and they therefore have large surface areas and high effective pore volumes (Pourhakkak et al. 2021).
The use of these adsorbents in wastewater treatment is however constrained by their cost, which make up about 70% of the overall cost of the adsorption process (Al-Ghouti and Da’ana 2020; Wang and Guo 2020). The cost of absorbents varies depending on the raw materials used to produce them (Sarafraz et al. 2019; Praveen et al. 2021), the method used to activate the adsorbent (Sakhiya et al. 2021), the type of chemical used for activation (Kani et al. 2020), and adsorbent selectivity, degradation rate, and the operational costs of producing the adsorbent (Luo et al. 2019; Ahmad et al. 2020). The processing (grinding, mixing, filtering, heating) of the raw materials used for adsorbent production and the amount of absorbate removed per unit area of absorbent are also some methods used to determine the cost of adsorbents (Ighalo et al. 2022; GadelHak et al. 2023). Studies by GadelHak et al. (2023) have shown that energy and the raw material used to produce an adsorbent could each contribute up to 90% of the total cost of producing the adsorbent. Raw material and energy cost are influenced by the type and source of the raw material, and the country where the material originates or where the biosorbent is being produced (Ighalo et al. 2022). This is because countries vary in their level of inflation and cost of energy. Where energy is expensive, the cost of converting raw materials to biosorbents may be higher than in countries with lower energy costs. In countries where the inflation rate is high, raw materials are also likely to be more expensive than in countries with low inflation rates. These prohibitive costs have triggered research into the production of low-cost adsorbents.
Absorbents classified as low-cost adsorbents are usually made from materials that are renewable, inexpensive, eco-friendly, require minimal or simple processing before usage, and are available in large quantities (Renge et al. 2012). Examples of such materials are agricultural wastes, clay, bentonite, and montmorillonite. The use of clays, bentonite, and montmorillonite as adsorbents is however limited by their low adsorption capacity (Kainth et al. 2024). This has necessitated investigations into the potential of agricultural wastes as low-cost adsorbents. In a study by Yasir et al. (2023) it was shown that using bagasse fly ash or rice husk fly ash instead of commercial activated carbon to remove 2,4-dichlorophenol from paper and pulp mill effluent resulted in an annual saving of US$6.33/m3 of water/year. This highlights the economic benefit of agricultural wastes as adsorbents. An analysis of the number of scientific articles published on Google Scholar in which investigations on the use of agricultural solid waste as adsorbents and as raw materials for adsorbent production are reported showed an increase from 1330 in 2012 to 263,000 in 2022. This increase highlights the growing interest in agricultural waste materials as low-cost adsorbents and raw materials from which such adsorbents can be produced.
Agricultural wastes with a potential to serve as adsorbents include fruit and vegetable wastes (FVW). World fruit and vegetable production has increased globally (Balali et al. 2020), with banana, durian, lemon, orange, potatoes and onions among the most widely consumed (Arias et al. 2022). Due to their high-water content, fruits and vegetables have a short shelf-life and therefore deteriorate within a short period of time when not well preserved. These spoilt fruits and vegetables together with their non-edible parts such as peels, pomace, seed, rind, residual stalks, straw, roots, leaves, and flowers comprise FVW. Peels are the outer covering or skin of a vegetable or fruit. According to Shakya and Agarwal (2019), Nguyen et al. (2022), Selvarajoo et al. (2022), Singh et al. (2022), and Manmeen et al. (2023), peels could contribute more than 50% of the overall weight of some fruit and vegetables, as shown in Table 1, depending on the peeling mode. Fruit and vegetable peels (FVPs) therefore comprise a significant fraction of agricultural solid waste. Kumar et al. (2020) and Rifna et al. (2023) reported that 90 to 92% of FVW are peels, while the remaining 8% is shared among seed, core, rag, stone, pods, vine, shell, skin, and pomace, among other plant parts. These FVPs are generated in massive quantities globally in household kitchens and in vegetable and fruit-based industries.
Table 1. Peels Weight Percentage in Selected Fruits and Vegetables
Modified after Shakya and Agarwal (2019), Nguyen et al. (2022), Selvarajoo et al. (2022), Singh et al. (2022) and Manmeen et al. (2023)
Joglekar et al. (2019), have reported that annual global fruit peel generation by countries follows the order China > USA > Philippines > India > Thailand > Malaysia, with approximate quantities of 35, 15, 8, 3, 2, and 1 million metric tons respectively. About 500 million tons of FVPs are produced by fruit and vegetable industries worldwide (Joglekar et al. 2019; Senit et al. 2019). They are therefore widely available and could be used as adsorbents or as raw materials for low-cost adsorbent production. Adsorbents produced from these plant-based materials as well as other biological materials are generally referred to as biosorbents. This review discusses the physico-chemical properties of both fruit and vegetable peels and biosorbents produced from them, the use of hydrothermal carbonization and pyrolysis to convert the peels to biosorbents, and the efficiency of the biosorbent produced from FVPs in the removal of pollutants from wastewater. The paper concludes with a discussion on the prospects of FVPs as biosorbents and raw materials to produce low-cost adsorbents.
FRUIT AND VEGETABLES PEELS AS BIOSORBENTS
FVPs contain various compounds and properties that make them capable of adsorbing various compounds. The authors’ analyses of published articles on Google Scholar that investigated FVW as biosorbents between 2012 and 2022 indicate that 32% of these articles (8640 articles) used FVPs. A few of these studies used these peels as adsorbents in their natural state. Pathak et al. (2016), for example, investigated the potential of the peels of pineapple, pomegranate, watermelon, garlic, and green pea as adsorbents and found that the surfaces of these peels are characterized by both acidic and basic sites, though the acidic sites were dominant in many of the peels. The peels also had lower surface areas relative to those of commercial adsorbents, but they contained similar functional groups, such as phenol, alcohol, carboxylic acid, alkanes, amines, amino acids, and aromatic alkyl halides, which could adsorb various contaminants. In another study by Ng et al. (2016), using the natural peels of sponge gourd as a biosorbent, malachite green was successfully removed from wastewater. Singh et al. (2018) also used powdered banana peel to effectively remove rhodamine-B from water. Other reports on the use of FVPs as adsorbents in their natural state are found in Reddy et al. (2015), Alvarez et al. (2018), Priyantha and Kotabewatta (2019), Ben-Ali (2021), Sánchez-Ponce et al. (2022), and Kainth et al. (2024). These studies all show that FVPs in their natural state have potential as adsorbents without having gone through any modification. However, when used as adsorbents in their natural state, they have low adsorption capacities (Yang and Jiang 2014; Olasehinde et al. 2018; Wattanakornsiri et al. 2022). They also have a high rate of biodegradation and could release soluble organic compounds, which may cause them to have low adsorption capacity, high chemical oxygen demand (COD), high biological oxygen demand (BOD), and high dissolved organic carbon (DOC) content (Wan Ngah and Hanafiah 2008; Adewuyi 2020; Kainth et al. 2024). Hence, chemically treated FVPs have also been investigated as biosorbents.
The most used method for the treatment of natural peels is the addition of a chemical to alter the surface properties of the peels. A few studies have reported on the methods and chemicals used in chemical treatment of FVPs in their natural state as well as the efficiency of the resulting adsorbents. Some of these studies and the methods used for treating the peels are presented in Table 2. Chemicals commonly used for treatment of these natural FVPs include H2SO4, FeCl3.6H2O, HNO3, and NaOH (Table 2). The data in Table 2 also show that one type of peel could be treated using different chemicals and methods.
Table 2. Chemicals and Processes Used for the Treatment of Various FVPs in their Natural State
With regards to the efficiency of chemically treated natural peel, Wattanakornsiri et al. (2022) used untreated peels of dragon fruit, rambutan, and passion fruit to remove Pb from water and found removal efficiencies of 76.6%, 49.6% and 84.7% respectively. The removal efficiencies were improved to 92.9%, 97.8% and 94.5% respectively for dragon fruit, rambutan, and passion fruit after treating them with 4 mol/L H2SO4 for 30 minutes. These researchers obtained similar improvements in efficiency with the same peels in the removal of Cd. Though chemical treatment improved the absorption capacities of the natural peels, their performance as adsorbents was not as good as the performance of their carbonized forms. In addition, the possibility of their degradation presents significant challenges to their use (Wattanakornsiri et al. 2022). Using natural peels of fruits and vegetables as raw material for adsorbent production instead of using them as biosorbents is therefore a more favored and widely researched use of FVPs. Among the FVPs that have been used as raw materials for the production of biosorbents are the peels of orange, banana, mango, avocado, mandarin, pineapple, tapioca, and litchi (Palma et al. 2016; Zhou et al. 2017; Shakya and Agarwal 2019; Wu et al. 2020; Qiao et al. 2021; Vigneshwaran et al. 2021a,b; Chen et al. 2022; Eleryan et al. 2022). The suitability of these peels as raw materials for biosorbents is determined by their physico-chemical properties, which dictate the surface characteristics and hence the sorption potential of the produced biosorbent.
PROPERTIES OF FVPs INFLUENCING THEIR SUITABILITY AS RAW MATERIALS FOR BIOSORBENTS
A plant’s chemical composition is generally determined by its degree of maturation (Rahman et al. 2016; Sabuz et al. 2020; Quamruzzaman et al. 2022), genetic factors and the cultivation methods used to grow the plant (Indulekha et al. 2017; Güzel and Akpınar 2020; Widayanti et al. 2023), and the environmental conditions under which the plant is cultivated (Urban et al. 2007; Drobek et al. 2020; and Christopoulos and Ouzounidou 2021). Differences in these factors mean that the same plant species may contain the same types of compounds and elements but in different concentrations because of the prevailing growth conditions. FVP properties of relevance to their potential as raw materials for biosorbent production are presented in Table 3 and discussed in the following sections.
Chemical Composition of FVPs
Like most plant parts, FVPs contain cellulose, hemicellulose, lignin, lipids, proteins, crude fiber, carbohydrate, alkaloids, hydroxides, carboxylic acids, alcohols, ketones, aldehydes, ether, and phenols, in amounts that vary with the type of fruit or vegetable (Table 3). These compounds contain polar functional groups and are rich in carbon (C) and oxygen (O). Carbon content in FVPs depends on the proportions of these compounds. Lignin has the highest ratio of carbon to oxygen (Demirbas 2003). Data presented in Table 3 show that all FVPs contain lignin, which contributes to a high amount of carbon. The rate of biodegradation of biosorbents with high C content would be slower, which can be explained by the content of lignin, which resists biodegradation. According to Pavlostathis (2011), lignin decomposition determines the rate at which lignocellulosic materials are biodegraded because of its high carbon and low nitrogen contents. Data presented in Table 3 indicate that FVPs contain C in the range of 39 to 52%, with mandarin peels having a higher amount of carbon compared to the other FVPs presented in Table 3. Biosorbents produced from mandarin peels may therefore be more stable than those from mango peels which have lower C content.
Oxygen is the primary element in many polar functional groups contained in compounds that are found in FVPs. These compounds contain both labile and recalcitrant O fractions, but the recalcitrant fraction is what is left in the biosorbent after carbonization of the peel (Tran et al. 2022; Viswanathan et al. 2023). FVPs contain a high amount of O, with concentration values ranging from 43 to 52.3% (Table 3). Durian peels have relatively lower O content than orange and lemon peels (Table 3). FVPs with high O content are likely to produce biosorbents with higher adsorption capacities because of the presence of many functional groups on the surface of the biosorbent. Nitrogen and sulfur, which are often associated with the formation of greenhouse gases are also present in all FVPs though in lower amounts compared to other elements (Table 3). Though not particularly important in peels used for biosorbents, they are important in peels used as feedstock for biochar designed for soil application.
Analyses of the elemental ratios in FVPs or any biomass provides some insight into their stability and degradability, polarity, and hydrophilicity (Hu et al. 2020; Wijitkosum 2022). High values of O/C and (O + N)/C respectively indicate a strong degree of hydrophilicity and polarity (Chen et al. 2016). FVPs are mechanically weaker than woody plants because they contain low amounts of lignin and are therefore easily biodegraded with consequences on their molecular structure (Abiodun et al. 2023). They are therefore not as stable as woody biomass and may not be reuseable, especially in their natural form (Fosso-Kankeu et al. 2014). Data presented in Fig 1 show that mango peels have weak aromaticity because of their high mean H/C values, whereas orange peels have the strongest aromaticity. According to Nzila (2018), aromatic compounds are more resistant to biodegradation than aliphatic compounds. Orange peel-based biosorbents are therefore likely to be more stable because of their higher aromaticity and slower rates of degradation compared to mango peel-based biosorbents.
Table 3. Chemical Composition of Selected Fruit and Vegetable Peel Biomass
