A review of the application of agricultural wastes as precursor materials for the adsorption of per- and polyfluoroalkyl substances: A focus on current approaches and methodologies

Environmental Technology & Innovation 9 (2018) 100–114

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Environmental Technology & Innovation journal homepage: www.elsevier.com/locate/eti

A review of the application of agricultural wastes as precursor materials for the adsorption of per- and polyfluoroalkyl substances: A focus on current approaches and methodologies Patricia N. Omo-Okoro, Adegbenro P. Daso, Jonathan O. Okonkwo * Department of Environmental, Water & Earth Sciences, Faculty of Science, Tshwane University of Technology, Private Bag X680, Pretoria, South Africa

highlights • • • •

Agro-waste materials employed as adsorbents for pollutants’ removal were appraised. An assessment of techniques for producing adsorbents from agricultural wastes is provided. Hydrophobic interactions are responsible for the adsorption of PFAS from aqueous media. Maize tassel-silver nanoparticles have the potential to remove PFAS from water systems.

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Article history: Received 8 June 2017 Received in revised form 16 November 2017 Accepted 16 November 2017 Available online 23 November 2017 Keywords: Agricultural wastes Adsorbents Removal Adsorption capacity Per- and polyfluoroalkyl substances

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a b s t r a c t There is an abundant body of literature surrounding the use of agricultural wastes for the adsorption of pollutants from aqueous solutions. This area of research is often considered as an alternative to conventional treatment techniques. In the past, the research focus centred on the adsorption of toxic metals. Presently, there is an increase in the calls, for researchers to explore new adsorbent materials for the removal of organic pollutants. Since once ingested, these organic pollutants have health impacts such as chronic kidney and liver diseases and endocrine disruption. Hence, there is an increasing need to discover new and efficient ways of removing organic pollutants from water systems. It is worth mentioning that most agricultural wastes are not usually utilized in their original state, but rather modified in diverse ways to increase the material’s surface area of adsorption and porosity. Thermal treatment, carbonization, chemical and physical activation, nanostructuring, grafting with copolymers and many others are some of the widely used methodologies; that are currently being employed for modifying agricultural waste materials for use as adsorbents. In this review, therefore, a discourse on a range of agricultural wastes that have been used as adsorbents for the removal of per- and polyfluoroalkyl substances (PFAS) from aqueous media is provided. A proposition of the use of maize tassel–silver nanoparticles as new comers is made. This review article serves to give key insights on the technical realizations in the area of adsorption of PFAS by utilizing agricultural wastes as precursor materials for preparing adsorbents. © 2017 Elsevier B.V. All rights reserved.

Correspondence to: Department of Environmental, Water and Earth Sciences, Faculty of Science, Tshwane University of Technology, Arcadia Campus, Pretoria 0001, South Africa. E-mail address: [email protected] (J.O. Okonkwo). https://doi.org/10.1016/j.eti.2017.11.005 2352-1864/© 2017 Elsevier B.V. All rights reserved.

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Contents 1.

2.

3.

4. 5. 6.

Introduction............................................................................................................................................................................................. 1.1. Per- and polyfluoroalkyl substances (PFAS) ............................................................................................................................. 1.1.1. Motivation — Why PFAS? ........................................................................................................................................... Agricultural-based adsorbents ............................................................................................................................................................... 2.1. Chitosan ...................................................................................................................................................................................... 2.2. Why should agricultural wastes be utilized? ........................................................................................................................... 2.3. Current approaches and methodologies for modifying agricultural wastes.......................................................................... 2.3.1. Carbonization and activation ..................................................................................................................................... 2.3.2. Agriculturally derived nanostructures and nanocomposites .................................................................................. 2.3.3. Grafting via copolymerization ................................................................................................................................... PFAS case studies .................................................................................................................................................................................... 3.1. Bamboo-derived granular activated carbon for PFAS adsorption........................................................................................... 3.2. Grape leaf litter activated carbon for PFAS adsorption............................................................................................................ 3.3. Grafting via ATRP and quaternization for PFAS adsorption .................................................................................................... 3.4. ATRP and amination for PFAS adsorption................................................................................................................................. 3.5. Burning and pyrolyzation for PFAS adsorption ........................................................................................................................ Challenges................................................................................................................................................................................................ Perspectives for future research ............................................................................................................................................................ Conclusions.............................................................................................................................................................................................. Acknowledgements ................................................................................................................................................................................ Conflict of interest................................................................................................................................................................................... References ...............................................................................................................................................................................................

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1. Introduction Modernization and increase in population have contributed to the continuous release of new and emerging chemical compounds into water sources (Kumar et al., 2009). Of all the persistent pollutants, certain organic compounds have been implicated in much of water pollution globally. A wide range of the implicated organic compounds have been found to exert toxic and detrimental effects on both the biotic and abiotic components of the environment. Therefore, the occurrence of organic pollutants in water has been an issue of utmost concern for the water industry at large, due to the acute toxicities, persistency and carcinogenicity of some of the pollutants (Ali et al., 2012). Generally, processes that have been utilized to remove chemical pollutants from different aqueous matrices include, but not limited to, membrane filtration, precipitation, ion exchange, solvent extraction and adsorption (Fu and Wang, 2011). Some of the drawbacks associated with these processes include: low removal efficiencies, high cost of regeneration, deposition of sludge, high energy demand, high reagent requirements, amongst others (Kumar et al., 2009). Multiple researches lend credence to the postulation that adsorption still remains one of the preferred techniques for pollutant removal from aqueous media since it is safer and easier to use. In a nutshell, adsorption involves an interaction between the outer surface of an adsorbent and that of a pollutant (Wahi et al., 2013). This interaction brings about the adhesion of the particles of the adsorbate on the surface of the adsorbent. A major quality of an adsorbent is the amount of adsorbate it can accumulate. This is usually calculated from the adsorption isotherms (Gupta, 2009). The adsorption process is dependent on several factors such as initial level of pollutant, solution pH, adsorbent dosage, rate of mixing and temperature. Absorption, on the other hand, entails the uptake of fluid into the matrix of a material. In this case, the sorbate’s molecules penetrate the material during absorption (Norizan et al., 2012). In recent times, the quest for complementary or alternative techniques for removing pollutants from the water system, with minimal environmental impacts has heightened. This quest has led to the employ of adsorption as a technique for pollutants’ removal. Adsorption is a widely used technology for organic pollutants removal from aqueous matrices. A wide variety of agricultural waste materials have been used for the preparation of activated carbons, and often times, these are referred to as ‘biosorbents’, also known as agricultural based-adsorbents (Crini, 2005; Yu et al., 2008; Zhou et al., 2011). Biosorption allows for the passive concentration and binding of pollutants on the cellular structure of the specific biomaterial (Volesky and Holan, 1995). They can be regenerated for multiple re-use (Volesky, 1990). Functional groups that have the potential to attract and sequester metals in agricultural based-adsorbents include, but not limited to; carboxyl and sulfhydryl groups, hydroxyls in polysaccharides, amino, amido, acetamido groups in chitin and phosphates in nucleic acids (Sud et al., 2008; Volesky and Holan, 1995). Over the years, the literature has been enriched with various applications which are based on the adsorption techniques for the removal of pollutants from aqueous waste streams (Adegoke and Bello, 2015; Malik, 2003; Radhika and Palanivelu, 2006). This vast body of knowledge has shown that agricultural-based adsorbents are promising

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alternatives to the conventional treatment techniques because of their inherent advantages such as minimization of chemical or biological sludge, high efficiency for dilute solutions, no additional nutrients requirements, ease of operation, possibility of adsorbent regeneration, and ultimately, the fact that these materials are non-hazardous to the environment contributes to their growing popularity (Jiménez-Cedillo et al., 2013). Adsorption has therefore, been embraced as a technique for pollutant removal due to the numerous advantages that come with its use (Akl et al., 2014; Lesmana et al., 2009). This clamour for the use of adsorption as a technique for depolluting aqueous environments can be attributed to the availability of biomaterials/biomass used as adsorbents, high adsorption capacities and the economic value of plant tissue (Lesmana et al., 2009). The research for adsorbents for the adsorption of organic pollutants from aqueous media has so far concentrated on dyes (Adegoke and Bello, 2015; Malik, 2003), oils (Ibrahim et al., 2010; Wahi et al., 2013) and polycyclic aromatic hydrocarbons (PAHs) (Crisafully et al., 2008; Li et al., 2010; Xi and Chen, 2014). However, the removal of POPs such as per- and polyfluoroalkyl substances (PFASs) from water systems has received little attention. With the exception of some long chain PFASs such as perfluorooctane sulfonate (PFOS) and perfluorooctanoate (PFOA) which have been listed in the Stockholm Convention for controlled usage; all other PFASs are still very much in use. Since some long and short chain PFASs have been shown to exhibit toxic effects on animal models, there is the need for their exclusion from the water value chain (Hu et al., 2002; Joensen et al., 2009; Lau et al., 2007). The aim of this review resides with the use of agricultural wastes for the preparation of adsorbents suitable for adsorbing PFAS from aqueous media. The toxicities of PFASs in various environmental matrices are also highlighted. Thereafter, some of the advances made within the last decade with a focus on the technique of adsorption of organic pollutants, particularly PFAS using adsorbents derived from agricultural wastes are expatiated on.

1.1. Per- and polyfluoroalkyl substances (PFAS) Per- and polyfluoroalkyl substances are substances that have some and/or all of their hydrogen atoms that are attached to a carbon chain length, being replaced by fluorine atoms (Rahman et al., 2014). They are represented by the moiety Cn F− 2n+1 (Rahman et al., 2014). Two of the most common PFAS are PFOS and PFOA. These compounds together with their salts have been listed under the Stockholm Convention as persistent organic pollutants (POPs), restricting their global use and production (Mudumbi et al., 2014). Following the prohibition of PFOS and its precursors, shorter-chained PFAS with less than seven carbon chains, particularly those belonging to the subgroups such as perfluorocarboxylic acids (PFCAs) and perfluorosulphonic acids (PFSAs) were introduced as alternatives. Per- and polyfluoroalkyl substances are mainly released into the environment and waterways when consumers use PFAScontaining products such as aqueous film forming fire-fighting foams (AFFF), stain repellents and cleaning agents, non-stick cook wares, protective coatings for carpets, textile and leather products, metal plating, food packaging materials, insecticide formulations, surface-active agents (surfactants) and casings for electrical wires (Mudumbi et al., 2014).

1.1.1. Motivation — Why PFAS? Per- and polyfluoroalkyl substances do not biodegrade, hydrolyse or photolyse under typical environmental conditions and, therefore, they are exceedingly persistent (Ahrens et al., 2015; Rahman et al., 2014). Per- and polyfluoroalkyl substances have the tendency to accumulate in the human blood and serum of human population and wild life (Giesy and Kannan, 2001; Zhang et al., 2010). Per- and polyfluoroalkyl substances have the potential to cause liver toxicity, affects the thyroid hormones, decrease in birth weight of new-borns, systemic effects, chronic kidney disease and prolonged exposure may cause cancer (Joensen et al., 2009; Post et al., 2009; Zhang et al., 2012; Moody and Field, 2000). Per - and polyfluoroalkyl substances pose many toxicity threats in animals, including low dose systemic effects (Benford et al., 2008). Joensen et al. (2009) reported a correlation between men highly exposed to PFOS and PFOA with men having impaired semen quality. Liver toxicity has been detected in primates after PFOS exposure. The PFOS molecules have been found to have the potential to slow/stop the in vitro gap-intercellular communications; in the liver and cell lines of rats exposed to PFOS for the purpose of laboratory studies (Hu et al., 2002; Lau et al., 2007). Developmental and reproductive toxicities are some of the endpoints associated with PFAS exposure (Benford et al., 2008). Per- and polyfluoroalkyl substances, therefore, have the potential to impair human health. Per- and polyfluoroalkyl substances have been identified in drinking water sources globally (Gellrich et al., 2013; Hölzer et al., 2008; Post et al., 2009; Skutlarek et al., 2006; Vierke et al., 2012). Per- and polyfluoroalkyl substances have been observed to be in the lower range of ng/L in freshwaters, in most parts of the world, while PFOS levels of >500 ng/L have been detected in effluents from wastewater treatment plant (WWTP) (Prevedouros et al., 2006). Lower ranges of ng/L in drinking water in many parts of Europe were recorded for PFOA in the studies by Loos et al. (2007), Haug et al. (2010), Ericson et al. (2008) and Thompson et al. (2011), where (1.0–2.9, Italy), (0.65–2.5, Norway), ((0.3–6.3, tap water); (<0.2–0.7, bottled water) in Spain) and (<0.5–9.7, Australia) were reported respectively. These data on the ubiquity, levels and toxicity of PFAS indicate that the exposure to PFAS is widespread. Based on these premises, their removal from any water system is, therefore, of importance.

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2. Agricultural-based adsorbents Agricultural based-adsorbents are solid materials of biological origin that have the potential to remove/adsorb pollutants from an aqueous medium. Algae, yeast, bacteria, fungi and lignocellulosic materials have proved to be potential adsorbents for the adsorption of toxic pollutants (Aksu, 2005; Lesmana et al., 2009). Agricultural based-materials include agricultural wastes and they may be referred to as agricultural by-products. These include wood bark, sawdust (powdery particles of wood produced by sawing), nuts, shells and husks of crops and plants, chitosan, starch and cellulosic fibres of crops and plants. They have been widely used as precursors for activated carbon preparation and as constituents of composite materials for the adsorption of organics (e.g. dyes, phenolics, pesticides) from aqueous media, with very limited studies on their use for PFAS removal. 2.1. Chitosan Chitosan is an amino polysaccharide that is biodegradable, biocompatible and possesses adhesive properties (Sogias et al., 2008). These characteristics are some of the core reasons why chitosan has been embraced as a promising adsorptive material, especially over the last decade. Chitin is the backbone structure for chitosan (Zhang et al., 2010), and it is an abundant, natural polymer found in the exoskeleton of arthropods, cell walls of fungi and some plants (Rinaudo, 2006). By virtue of this, chitin and chitosan can be seen as agricultural-based materials. Perfluoroctane sulfonic acid has shown its affinity to accumulate in the blood plasma and kidney than in the fatty regions of the animal body (Kannan et al., 2002). Hence, the amino groups present in chitosan may act as an attracting/binding force to PFOS. In other words, the amino units inherent in chitosan can be protonated in acidic solution and thereby used for the adsorption of anionic PFOS (Zhang et al., 2011). In the study by Zhang et al. (2011), chitosan beads were used as an adsorbent for PFOS. The chitosan beads were formed by the addition of a specific mass (1.5 g) of chitosan powder in acetic acid (50 mL of 2% (v/v)) and sodium hydroxide (NaOH) (500 mL; 0.5M). The addition of the acetic acid and NaOH influenced the formation of the chitosan beads from the chitosan powder. Epichlorohydrin (ECH) was used as a crosslinking agent. The ECH application prevented the solubility of the prepared chitosan beads. An adsorption capacity of 5.5 mmol/g for PFOS was reported using chitosan beads as an adsorbent. From the results presented in the study, there is an indication that PFOS had been adsorbed on the chitosan beads after adsorption experiments. This conclusion is reached due to the presence of FTIR peaks reported on the chitosan beads after its use for adsorption of PFOS (Zhang et al., 2011). These peaks represent both sulfonic groups and C–F bonds synonymous with PFOS. Though chitosan may be regarded as a good adsorbent as shown in the study above, however, the process involved the use of numerous solvents such as CH3 COOH, NaOH and ECH. These solvents are not cheap despite the fact that chitosan itself is produced from the chitin found in cell walls of some plants and arthropod. Furthermore, the study was performed on a laboratory scale and not on real PFAS contaminated samples. Yu et al. (2008) recorded (560 µmol/g) as the adsorption amount for PFOS using chitosan-based molecularly imprinted polymer adsorbents. It should be noted that from both studies, that the formation of PFOS micelles on the chitosan adsorbent enabled the high adsorption capacities reported; as electrostatic and hydrophobic interactions were seen to be at play during the adsorption processes. 2.2. Why should agricultural wastes be utilized? Agricultural wastes, especially those derived from plant biomass (e.g. husks, tassels, cobs, straw, shells, peels, amongst others) that can be otherwise discarded usually after harvesting are widely used as precursor materials for the production of adsorbents. Some of these have been employed for the removal of pollutants from aqueous media, including: maize cob, maize tassel, rice husk (Zvinowanda et al., 2009; Olorundare et al., 2014; Deng et al., 2013), rice hull, cocoa shells, black gram husk, almond husk, sugar-cane bagasse (Fisal et al., 2011; Akl et al., 2014), sugar-beet pectin gels, coconut shells and coconut husks (Bhatnagar et al., 2010), mangosteen shell, carrot residues, barley straw, cassava and tea wastes, banana (Oyewo et al., 2016, 2017), orange and citrus peels amongst others (Malik, 2003; Ali et al., 2012). Most plant wastes are usually not used in their raw forms without some forms of pre-treatments, but are often subjected to either physical or chemical modification processes. These are often necessary in order to improve or enhance the surface properties of the resulting adsorbent materials. Previous studies have shown that chemical modifications of plantbased waste materials with oxygen, nitrogen and sulphur containing groups and with other chemically modifying agents can significantly enhance the adsorption and cation exchange capacities of the resulting adsorbent materials (Wartelle and Marshall, 2006). In practice, chemical modification may involve acetylation, acylation, methylation and benzoylation reactions. Furthermore, modification may also entail the physical conversion by heating at elevated temperatures of the agricultural wastes into activated carbons. Li et al. (2010) utilized modified pine bark as an adsorbent. The study showed that the modified pine bark possessed the potential to adsorb PAHs. Pine bark contains constituents such as polysaccharides, suberin, lignin and lipid which help to improve its adsorption capacity. The physical activation of agricultural wastes by carbonization, for instance, increases their surface area for adsorption, pore size distribution (PSD) and total pore volume. Some studies have shown a number of cases whereby activated carbons prepared from agricultural wastes performed better than the commercially available activated carbons (Bhatnagar et al., 2010; Radhika and Palanivelu, 2006; Wang et al., 2015). These support the fact that low cost modification techniques may serve as an added advantage and does not alter the innate

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advantages such as eco-friendliness, renewability, availability, biodegradability, reusability and efficiency that comes with utilizing plant-based agricultural wastes as important precursor materials for adsorbent production. Some of the basic requirements of a good adsorbent include easy desorption, negligible release of unexpected substances into aqueous solutions and high regeneration capability (Nguyen et al., 2013). Commonly used agricultural wastes as adsorbents in the past years have been documented in a number of published papers (Lesmana et al., 2009; Aksu, 2005; Ali et al., 2012; Bhatnagar et al., 2010). Most of these are mainly employed for the removal of heavy and trace metals in aqueous media (Kanu et al., 2016; Olorundare et al., 2012; Zvinowanda et al., 2009) thereby leaving a gap on information on their suitability for the removal of organic pollutants from similar matrices. 2.3. Current approaches and methodologies for modifying agricultural wastes As previously stated, plant-based waste materials are not often employed in their raw forms as adsorbent materials but may be subjected to some forms of modifications. In this section, we critically appraise some of the current approaches and methods of modifying these agricultural wastes, including thermal treatment, carbonization, activation, grafting, nanostructuring, and many others. 2.3.1. Carbonization and activation Activated carbons are highly porous carbonaceous materials that have been profoundly utilized for chemical purification, as adsorbents, catalysts and catalyst supports (Zhang et al., 2004; Mohan and Pittman, 2006; Dias et al., 2007). Research has shown activated carbon to be an ideal material for removing organic pollutants from aqueous media. This is due to its large surface area, high adsorptive capacity and its high reactivity. However, the use of commercially available activated carbon is not cost-effective. The search for low cost activated carbons began about a decade or two ago, as an alternative for the coalbased activated carbon. Agricultural-based activated carbon refers to the use of agricultural residues/wastes as precursors for producing activated carbons. Coconut shells, peach pits, rice hulls and husks, fish, peat, wood char, sugar bagasse and many other agricultural waste materials have been used in preparing activated carbon (Mohan and Pittman, 2006; Torrellas et al., 2015). The selection of ideal materials is usually based on availability, low degradation processes upon storage and the ease of activation (Torrellas et al., 2015). Producing activated carbons usually entails carbonization, pyrolysis and activation (either chemical or physical) (Zhang et al., 2004; Olorundare et al., 2014). Pyrolyzation involves the irreversible application of heat at elevated temperatures in the absence of oxygen or any halogen. This in turn, causes the thermal decomposition of organic material(s) and thereby converts the material to carbon, i.e., carbonization (Zhang et al., 2004). Activation can be performed by physical or chemical means. Carbonization helps to enrich the carbon content of the material while activation allows widening of the pores, thereby making the material more porous (Daud and Ali, 2004; Olorundare et al., 2014). Physical activation usually entails the burning of some of the raw carbon content of the agricultural waste material in the presence of oxidizing agents. Oxidizing agents that are frequently utilized for physical activation include carbon dioxide (CO2 ), steam, air or their combinations. Physical activation usually occurs at high temperatures, for example, 700 ◦ C in steam and CO2 , but at lower temperatures in air (Zhang et al., 2004; Demiral et al., 2011). These aforementioned oxidizing agents that are utilized during physical activation are arguably clean and easy to handle, with barely any hazardous environmental impact. However, one of the concerns associated with thermal treatment/thermal activation, i.e. application of heat alone, and also physical activation is the tendency of some agricultural waste materials to burn with increasing temperature leading to a significant decrease in yield (Shinogi and Kanri, 2003). Chemical activation, on the other hand, involves the addition of activating agents such as phosphoric acid (H3 PO4 ), zinc chloride (ZnCl2 ), NaOH, potassium hydroxide (KOH), and others. Some of the advantages of the chemical activation technique include lower temperature of activation, reduced energy costs, higher yield of material and less time of reaction (Dias et al., 2007). Nonetheless, this technique has its own drawbacks which include the possibility of generating secondary environmental pollution during the disposal of the used material (Zhang et al., 2004), and also the use of the activating agents employed for chemical activation is not cost-effective. A discussion of case studies centred on the use of carbonization and/or activation as techniques for modifying agricultural waste materials is provided below. Liu et al. (2010) reported the preparation of a bamboo-derived activated carbon via microwave-induced phosphoric acid activation. Optimized conditions that allowed the synthesis include phosphoric acid/carbon ratio 1:1, radiation time 20 min and power, 350 W. A surface area of 1432 m2 /g and a carbon yield of 48% were reported for the obtained agricultural waste-activated carbon. A comparison was made in the study between (i) heating conventionally using furnace, i.e., thermal activation in the presence of phosphoric acid as an activating agent (600 ◦ C, phosphoric acid/carbon ratio (1:1), activation time 30 min and 60 min); and (ii) heating using microwave (350 W, radiation time 20 min and phosphoric acid/carbon ratio (1:1)). It was found that the microwave induced chemical activation resulted in a faster activation rate, a bamboo derived activated carbon with a wider surface area and a higher carbon yield. The microwave technique also utilized less energy. Another effect of microwave irradiation on the material is the development of the pore structure. The study reported a poorly developed pore structure at low microwave power. An improved pore structure development was observed with increase in microwave power. It can be deduced from the study that there is a direct relationship between mesoporosity and microwave power. This is because the study reported a better development of mesopores with increased microwave power. Mesopores were also more readily formed with increased radiation time. The physical properties and chemical composition of the precursor (agricultural waste material), the carbonization and activation methods jointly determine the pore size

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distribution, surface area and porosity characteristics of the produced activated carbon (Mohan and Pittman, 2006; Demiral et al., 2011). The underpinning effect(s) of carbonization and activation techniques on the adsorbent is the rearrangement of the pore structures of these agricultural waste materials. The chemical activating agent, phosphoric acid facilitates the deposition of acidic groups on the carbon surface, thereby functionalizing it. The authors confirmed this process via Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The presence of acidic groups on the surface of an adsorbent is favourable for the adsorption of organic pollutants or for other applications where acidic groups are desired (Liu et al., 2010). Zhang et al. (2004) also conducted a physical activation of corn hulls in the presence of CO2 . In that study, the authors performed a comparison between the material obtained after pyrolysis (thermal treatment only) and that obtained after both thermal treatment and CO2 activation. It was found that the surface area, total pore volume and micropore volume of the CO2 activated material were significantly higher than those of the materials that were only subjected to thermal treatment. The corn hulls-CO2 -activated carbons had a Brunauer Emmett Teller (BET) surface area of 977 m2 /g, more than twice of that of the only-thermally treated corn hulls (411 m2 /g). Carbon dioxide has the ability to open closed pores and also expand pre-existing pores, thereby increasing the accessibility of the pores to the molecules of any adsorbate (Rodriguez-Reinoso et al., 1995); which was clearly demonstrated by Zhang and his co-workers (Zhang et al., 2004). Another observation from that study was the absence of the use of robust surface characterization techniques such as scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and others, to provide key information on the microstructure and functional groups inherent in the produced corn hulls-activated carbon. The only characterization performed in the study was BET. Demiral et al. (2011) reported the production of activated carbon from olive bagasse. The olive bagasse was thermally treated (heating only) at 500 ◦ C in nitrogen atmosphere to produce olive bagasse char. This was followed by activation with steam (i.e. physical activation) at higher temperatures ranging from 750–900 ◦ C for 45 min. to produce olive bagasseactivated carbon. The BET surface area reported ranged from 523–1106 m2 /g, with the highest surface area recorded for the activated olive bagasse at >750 ◦ C. Characterization techniques such as SEM and FTIR were performed in the study to elucidate on the surface characteristics of the produced activated carbon. From the study, there was a clear difference in terms of surface morphology, pore structure and surface area for the (i) the thermally treated (heating only) olive bagasse i.e. olive bagasse char; (ii) olive bagasse-activated carbon (750 ◦ C) and (iii) olive bagasse-activated carbon (750–900 ◦ C). This was evident in the SEM images reported for (i), (ii) and (iii) mentioned above. Where a dense and ‘absence of pores’ structure represented the olive bagasse char, a wavy outline with fairly developing pore system for the olive bagasse-activated carbon (750 ◦ C) and a well-developed pore structure of different sizes of pores for the olive bagasse-activated carbon (>750 ◦ C). It was evident that the varying temperatures influenced the pore structures. It was observed from the reported FTIR spectra that there were losses and additions of peaks for (i), (ii) and (iii) mentioned above indicating changes in the functional groups inherent in the produced materials. For instance, the peak at 3637 cm−1 representing O-H stretching was vivid in the FTIR spectrum of the olive bagasse char but absent in the FTIR spectra of both olive bagasse-activated carbons (750 and >750 ◦ C). Generally, it can be deduced from the study discussed, Demiral et al. (2011) and other similar studies e.g. Guo and Lua (2002), that physical activation is a step-ahead the thermal treatment technique. It is also evident that an increase in activation temperature can create more pores, increases the total pore volume, micropore volume, and ultimately the BET surface area of agriculturally-derived activated carbons. A maize tassel derived activated carbon was produced via chemical activation using H3 PO4 in the study by Olorundare et al. (2014). In that study, for the determination of surface area, crystallinity, morphological information and surface functional groups by BET, XRD, SEM and FTIR, respectively, were performed. The study reported a high surface area of 1263 m2 /g at the impregnation ratio 1:4 (precursor to phosphoric acid ratio). In the study, the surface area, crystallinity, presence of acidic groups and porosity increased with increase in impregnation ratio. While a large surface area was achieved, the possibility of the chemical activating agent (H3 PO4 ) to constitute secondary waste should be taken into consideration. The aforementioned case studies have attempted to expatiate on diverse techniques of producing activated carbons, namely: thermal treatment, carbonization and activation (physical/chemical) techniques. Furthermore, it is also evident from published data that there have been several attempts to produce activated carbons from agricultural wastes, which were successfully applied for the removal of organic pollutants. Birgani et al. (2016) was able to reduce considerably, excess amounts of chemical oxygen demand (COD) contained in batik (textile dye) wastewaters to <50 mg/L at a pH of 3, using palm shell-activated carbon. AntimaKatiyar and Sharma (2014) also investigated the possibility of removing metal cutting fluids from industrial waste water. This was achieved with the use of pumpkin seed waste-activated carbon. In a related study by Ibrahim et al. (2010), barley straw was chemically modified by a cationic surfactant, hexadecyl pyridinium chloride monohydrate (CPC) and was used to remove emulsified canola oil from aqueous solution. The addition of CPC to the barley straw’s surface created a non-polar layer on the surface of the barley straw thus endowing the surfactant modified barley straw (SMBS) with a much better adsorption capacity for oil removal from water. The surface properties of barley straw such as the availability of specific functional groups like hydroxyl (−OH) allowed for the easy modification of its surface. The adsorption capacity decreases with decreasing pH. The SMBS was effective in removing oil and the adsorption capacity of the SMBS reported in the study by Ibrahim et al. (2010) was 576 ± 0.3 mg/g at 25 ◦ C. Walnut shell has the ability to remove oil to very low concentrations; straining off oil without binding with it or holding on to it (Owens and Lee, 2007). During adsorption of oil using biomass, there is usually the possibility of the existence of both mechanisms — adsorption and absorption (Norizan et al., 2012). Potassium hydroxide was used to transform rice straw waste biomass into activated carbon in the study by Chang et al. (2012), and the resulting activated carbon was used to remove Bisphenol A (BPA) from an aqueous solution. The rice

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straw-activated carbon removed BPA rapidly and effectively. Radhika and Palanivelu (2006) prepared activated carbon from coconut shell. The coconut shell-activated carbon showed higher removal efficiencies, 99.9% and 99.8% for parachlorophenol (PCP) and 2,4,6- trichlorophenol (TCP), respectively, from aqueous solutions than commercially obtained activated carbon (97.7% and 95.5% for PCP and TCP, respectively). The coconut shell-activated carbon performed better than the commercially obtained activated carbon probably due to higher BET surface area (935.24 m2 /g) reported in the study for coconut shellactivated carbon. The coconut shell-activated carbon prepared via KOH activation (50%) had the highest BET surface area (935.24 m2 /g). The study was further applied to real environmental samples where pulp and paper mill effluent obtained from Karur, India were employed. A notable observation from the study is the use of many chemicals as activating agents e.g. KOH, NaOH, calcium carbonate (CaCO3 ), potassium permanganate (K2 MnO4 ) and ZnCl2 . Sharma et al. (2008) investigated the potential of coconut shell charcoal and coconut fibre alongside other adsorbents for the adsorption of atrazine from water. The removal efficiency for the removal of atrazine from water was 92.4%–95.2% using coconut shell charcoal and 85.9%–86.3% using coconut fibre. The study revealed that though coconut shell charcoal and coconut fibre do not possess wide surface areas, they were still very effective in removing atrazine. It is evident from cited instances above and from those that abound in literature that many agricultural wastes have successfully been utilized for the preparation of agro-based-activated carbons with high adsorption capacities to remove a number of organic pollutants, however, only a few of these have been applied to PFAS. 2.3.2. Agriculturally derived nanostructures and nanocomposites Nanotechnology involves the structuring and manufacturing of materials at a nanoscale (atomic and molecular scale) (Farokhzad and Langer, 2009). Nanoscaled structures refer to structures that fall within the 1–100 nm size regimes. Nanotechnology has been applied in numerous areas, including medicine and drug delivery (Farokhzad and Langer, 2009), food sector (Singh et al., 2016), textile industry (Kaounides et al., 2007), water purification and wastewater treatment (Li et al., 2008; Qu et al., 2013). Metal nanoparticles have been attached on various materials such as carbon nanotubes, activated carbons, agricultural waste biomasses, chitosan and other polymers (Saifuddin et al., 2011; Kumar et al., 2012; Djerahov et al., 2016). For instance, Kumar et al. (2012) was able to synthesize a nanocomposite using silver nitrate and an agricultural waste, Annona squamosa (sugar apple). Metal nanoparticles such as silver and gold have the ability to increase the surface atoms and invariably the surface energy of the substrate on which they are embedded on (Saifuddin et al., 2011). These resulted in increased functionality and reactivity. In addition, silver nanoparticles have also been reported to possess antibacterial activity when used for water purification purposes (Motshekga et al., 2013, 2015). A major concern with the technique of silver nanoparticle decoration of agricultural waste materials is the tendency of silver to leach beyond the acceptable level of silver (0.1 mg/L) (WHO, 2011) when used for water treatment. Beyond the maximum permissible limit, silver may be cytotoxic to both normal and cancer cells (Hussain et al., 2005). Hence, the performance of adequate leaching studies during preliminary studies is encouraged. Oyewo et al. (2016) employed the technique of nanostructuring via the mechanical milling of wet banana peels into nanosizes. The produced banana nanosorbent was used for the adsorption of actinides (uranium and thorium) in Oyewo et al. (2016) and lanthanides (lanthanum) (Oyewo et al., 2017) from synthetic and real mine water. In the study (Oyewo et al., 2016), the milling procedure over different durations (time) led to a reduction in particle size from <65,000 nm to <25 nm and a reduction in crystallite size from 108 to 12 nm. Another effect of milling on the adsorbent is the fracturing of particles resulting in a surface area increment from 1.07 to 4.55 m2 /g (Oyewo et al., 2017). The derived banana nanosorbent was characterized via FTIR, SEM, XRD and zeta sizer nano series. These characterization techniques facilitated the elucidation of the surface morphology of the agriculturally derived nanosorbent. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was used to determine the residual concentration of radioactive substances after the adsorption experiments. Langmuir adsorption capacities of 27.1 mg/g and 45 mg/g for uranium and thorium, respectively, from the synthetic mine water were reported. The adsorption capacities for uranium and thorium were 34.13 mg/g and 10.10 mg/g, respectively, for the real mine water. An adsorption capacity of 47.8 mg/g was recorded for lanthanum from mine water. It was observed from the study that chemicals such as NaOH and nitric acid (HNO3 ) were used to pre-treat the banana peels before milling to improve the adsorption capacity of the materials. However, a school of thought may regard this action as ‘a step away’ from the much-heralded green technology. The applicability of the study for real mine water and not just synthetic mine water is one of the good points of the study. 2.3.3. Grafting via copolymerization Grafting of polymers onto substrate surfaces is one of the recently used methodologies for functionalizing (adding functional groups) on fibrous materials such as agricultural waste materials (Cabane et al., 2016). The act of grafting polymers on adsorptive materials increases the density of adsorption sites on the adsorbent surfaces and thereby enhances the rate of adsorption of the adsorbates (Deng et al., 2012). Cotton waste was chemically modified and converted to a copolymercellulosic adsorbent in the study by Abdel-Halim and Al-Deyab (2012). This was achieved by treating the cotton waste with K2 MnO4 , subsequently bleaching it and, thereafter, grafting methacrylic acid onto it. The derived polymethacrylic acidcellulose graft copolymer (PMAAC) was utilized as an adsorbent for the removal of divalent cations (copper, nickel and cobalt ions) from aqueous solutions. Grafting of natural cellulosic wastes with suitable chemicals enhances the functional properties of the materials (Abdel-Halim et al., 2008; Abdel-Halim and Al-Deyab, 2012). An adsorption capacity of 250 mg per 1 g of PMAAC was reported in the study. One of the limitations of the reported study is its applicability on a laboratory

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Table 1 Some types of agricultural wastes used as precursor materials for pollutants removal. Agricultural waste-adsorbent

Modification

Pollutant(s)

Reaction conditions

Concentration range tested

Adsorption capacity (AC)/ Removal

References

Palm pith-activated carbon

Carbonization at 700 ◦ C for 1 h

2,4 –dichlorophenol (2,4 –DCP) from aqueous solution

10 mg/L

AC; 19.16 mg/g and 95% removal

Sathishkumar et al. (2007)

i. Green coconut shells. ii. Sugar cane bagasse

Lyophilization (freeze-drying i.e. freezing and dehydration of the frozen material under high vacuum

PAHs conc; (5.0–15.0 mg/L).

Maximum ACs reported; green coconut shells — 0.553 mg/g; sugar cane bagasse — 0.345 mg/g, both had higher ACs than that of chitin, chitosan and silica

Crisafully et al. (2008)

Cocoa (Theobroma cacao) shell-based activated carbon

Cocoa shell pellets were carbonized at 800 ◦ C and then physically activated using CO2 Oven drying; 80 ◦ C

Polycyclic aromatic hydrocarbons (PAHs; Pyrene, anthracene, acenaphthene and naphthalene) from petrochemical wastewaters. 4-Nitrophenol (4-NP) from aqueous solutions

Particle size; 250–500 µm; adsorbent dosage; 0.1 g; agitation; 20 000 rpm for 20 m; Adsorbent dosage; 0.20 g; agitation; 150 rpm for 10 h; pH; 7.5

Adsorbent dosage; 0.4 g100 mL of dye solution; agitation; 120 rpm; 30 ◦ C Isotherm experiments were performed at 20 ◦ C in a bath shaker 50 mL aqueous solution of phenol; pH range (2–12); temp (17–55 ◦ C); agitation; 125 rpm; adsorbent dosage (0.01–0.2 g) 30 mL of aqueous solution of BPA; adsorbent dosage; 0.1 g of treated adsorbents; agitation: 150 rpm; 24 h

100 mg/L

98% of 4-NP was removed/adsorption capacity was 167.17 mg/g

Fisal et al. (2011)

Range; 10–1000 mg/L

McKay et al. ACs; (875 and (1999) 277) mg/g for Safranine and Methylene Blue, respectively Akl et al. (2014) Sugarcane bagasse-steam activated carbon; 46.43mg/g;Sugarcane bagasse-NaOH activated carbon; 101 mg/g

Cotton waste

Sugarcane bagasse (SCB)-activated carbon

(i) Coconut husks (coir pith)

Basic dyes; (i) Safranine and (ii) Methylene Blue

Phenol from natural water samples

Carbonization (700–900 ◦ C) followed by physical activation using steam and also via chemical activated using NaOH Treatment of the adsorbents with sulfuric acid

Bisphenol A (BPA) from aqueous solutions

Treatment with acetic anhydride

Crude oil from aqueous medium

(ii) Durian peel

Range; 100–1600 mg/L

20 mg/L

(iii) Coconut shell Corncobs

Adsorbent dosage; 1 g

5g of crude oil displaced in 100 mL of water

Coir pith; 72% of BPA was removed with AC of 4.308 mg/g. Coconut shell (69%; 4.159 mg/g). Durian peel (70%; 4.178 mg/g) Acetylated corncobs; 0.0768 mg/g Raw corncobs; 0.0043 mg/g

Lazim et al. (2015)

Nwadiogbu et al. (2016)

scale only, i.e. on synthetic cations-containing solution and not on real aqueous environmental samples. Additionally, many chemicals such as sodium chlorite (NaClO2 , NaOH, sodium carbonate (Na2 CO3 ), K2 MnO4 , potassium iodide (KI), sulfuric acid (H2 SO4 ) and methacrylic acid were utilized. These chemicals may constitute secondary wastes. Even though cotton waste is abundant and readily available, the use of numerous solvents and chemicals is not cost-effective and may not be safe for the environment. Atom transfer radical polymerization (ATRP) is a broad and diverse polymerization technique that can be used for grafting polymer brushes (a surface coating of polymers) onto various organic and inorganic materials (Barbey et al., 2009; Morandi et al., 2009). Atom transfer radical polymerization has been used in conjunction with other methodologies such as amination (introduction of amine groups onto an organic molecule) and quaternization (creating a central positively charged cation/nitrogen atom with four substituents) for the modification of agricultural wastes for adsorption purposes (Deng et al., 2013). In addition to the highlighted examples of studies where agricultural wastes have been modified; a summary table showing the use of agricultural wastes as precursor materials following treatment or modification(s) is shown in Table 1.

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3. PFAS case studies In this section, we attempt to appraise some adsorption techniques which are specifically focused on the removal of PFAS from aqueous media. In general, it appeared that most published data on the use of agricultural wastes as precursor materials for the adsorption of PFAS show that many of the studies have been applied only on a laboratory scale. Only scanty information is available in literature on the up-scaled applications for PFAS removal in real environmental matrix. 3.1. Bamboo-derived granular activated carbon for PFAS adsorption Bamboo derived granular activated carbon (via KOH activation) was used for the adsorption of PFOS and PFOA (Deng et al., 2015). The bamboo was sourced locally from a market and then crushed and sieved into a particle size range of 0.85– 0.60 mm. The bamboo particles were then subjected to carbonization and chemical activation using KOH. Carbonization of the bamboo particles entailed the heating of the bamboo particles at a high temperature of 500 ◦ C under N2 flow in a furnace. This was followed by KOH activation at a mass ratio of KOH to carbon (1:4). The obtained material was dried at 105 ◦ C for 12 h and then further heating was performed at activation temperature of 700–900 ◦ C. The obtained adsorbent (bamboo derived granular activated carbon) was found to have large sized pores. It was used for the adsorption of PFAS via batch equilibrium experiments. Adsorbent dosage utilized in the study was 0.01 g. Agitation was carried out in an orbital shaker for 48 h at 175 rpm. The initial PFOS and PFOA concentrations were 100 and 81 mg/L, respectively. The maximum adsorbed amount (mmol/g) of 2.32 and 1.15, respectively, was reported for PFOS and PFOA. The enhanced adsorption of PFAS reported in the study may be due to the presence of mesopores and the large surface area (2450 m2 /g) recorded for the bamboo derived granular activated carbon. Deductions from the study by Deng et al. (2015) include; (i) hydrophobic interaction was majorly responsible for the adsorption process; (ii) the adsorption of PFOS and PFOA decreased with increasing solution pH; (iii) commercially available GACs usually contain mainly micropores which are unfavourable for PFAS diffusion and adsorption into the inner pores of the GAC. This may be due to the fact that micelles and hemi-micelles of PFAS that form during adsorption have the tendencies to block the micropores of the commercially available GACs; thereby rendering some inner surfaces of the GAC unavailable for adsorption to take place, and (iv) bamboo-derived GAC possesses the potential to adsorb PFAS under optimum conditions. The study was performed on a laboratory scale, i.e., the bamboo derived adsorbent was not applied for the removal of PFAS in real contaminated effluents. 3.2. Grape leaf litter activated carbon for PFAS adsorption The adsorption of PFOA and PFOS from aqueous solutions using the waste material of Vitis vinifera (Grape) leaf litter was investigated by Fagbayigbo et al. (2017). Activated carbons were prepared from the grape leaf litter via chemical modification. The grape leaf litter were washed and air-dried (48 h) and then oven-dried (80 ◦ C; 12 h). This served to expel all moisture therein. The pulverized and sieved leaf litter powder was thermally treated at high temperature (450, 600, 750 and 900 ◦ C) to carbonize (convert to carbon) the agricultural waste material. The obtained charcoal after thermal treatment was chemically activated using KOH and H3 PO4 . The study reported that there were a large number of micropores and mesopores on the surfaces of the derived activated carbons. This could be due to the use of H3 PO4 as an activating agent (Olorundare et al., 2014). The untreated grape leaf biomass and the grape leaf-activated carbon (before and after use) were characterized via FTIR, SEM, EDS and BET. The study reported maximum adsorption capacities of 78.90 and 75.13 mg/g for PFOA and PFOS, respectively, for the activated carbon treated with H3 PO4 . It can be inferred from the study by Fagbayigbo et al. (2017) that both the activated carbon prepared under acidic conditions from the grape leaf litter (AC-H3 PO4 ) and the activated carbon prepared under basic conditions from the grape leaf litter (AC-KOH) performed reasonably well, with AC-H3 PO4 showing higher removal efficiencies of 95 and 90% for PFOA and PFOS, respectively. The high carbon content (62.13%) and large surface area (295.48 m2 /g) recorded for the grape leaf litter activated carbon (AC-H3 PO4 ) could be responsible for the remarkable adsorption capacity that was reported for the AC-H3 PO4 . In addition, numerous studies (Elizalde-González and Hernández-Montoya, 2007; Fernandez et al., 2014) have lauded the use of H3 PO4 as a good modifying agent for agricultural waste adsorbents, especially those with lignocellulosic contents. Using H3 PO4 at increased temperatures generates a high surface area and increased porosity (Marsh and Rodriguez-Reinonso, 2006; Torrellas et al., 2015). However, the grape leaf litter activated carbon was only applied for the adsorption of PFAS from simulated or synthetic aqueous solutions. 3.3. Grafting via ATRP and quaternization for PFAS adsorption Fibrous cotton was used to prepare an adsorbent for the adsorption of PFOS and PFOA via surface initiated ATRP and quaternization by Deng et al. (2012). The ATRP initiators, bromoisobutyryl bromide (BIBB) were initiated onto the cotton surface. This was followed by the grafting of polymer brushes of poly (2-dimethylamino) ethyl methacrylate (PDMAEMA) via ATRP to produce the cotton copolymer (cotton-g-P (DMAEMA)). The ATRP technique allowed for the grafting of polymer brushes on the cotton surface thereby functionalizing the cotton material with useful functional groups. This was followed by the addition of bromoethane (2 mL) and tetrahydrofuran (1 mL) onto the derived cotton-g-P (DMAEMA) to produce quaternized cotton adsorbent. This quaternization technique served to convert the inherent neutral amine groups to

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quaternary ammonium cations. This also aided in functionalizing the newly derived cotton adsorbent. Grafting of polymers onto substrate surfaces is one of the recently used methodologies for functionalizing (adding functional groups) on fibrous materials (Cabane et al., 2016). The grafting of polymers with functional groups increases the adsorption velocity of adsorbates on the surfaces of the adsorbents and also widens the range of adsorption sites on the surfaces of the adsorbents (Deng et al., 2012). Initial PFAS concentrations reported in the study were 0.46 mmol/L for each of PFOA and PFOS. The adsorbent dosage utilized for batch equilibrium experiments during the adsorption process was 0.01 g. Maximum adsorption capacities of 3.3 and 3.1 mmol/g at pH 5.0 for PFOS and PFOA, respectively, were reported in the study. A number of inferences that supports the notion that quaternized agricultural waste materials (via ATRP) are promising adsorptive materials for PFAS can be deduced from the study. Firstly, the XPS analysis performed in the above study verified the presence of grafted quaternary ammonium-bearing long brushes and anionic groups (from the PFAS) on the surface of the modified cotton material after the adsorption process. The XPS analysis, therefore, helped to indicate that anionic PFOS and PFOA were adsorbed on the functionalized cotton adsorbent. Secondly, the spectra obtained from the FTIR analyses in the study showed that the peaks of the quaternary ammonium shifted after PFOA and PFOS adsorption. This was in agreement to the suggestion by Deng et al. (2012) that anion exchange played a key role in the adsorption of PFAS on quaternized cotton. Though, the utilization of quaternized agricultural waste adsorbent may bring about enhanced adsorption of PFAS from aqueous media; the use of too many chemicals such as BIBB, copper(I) and copper(II) bromide, PDMAEMA, bromoethane and tetrahydrofuran is a cause for concern. In addition, this study was employed for the adsorption of PFAS from simulated PFAS water. There is no documented report yet on the use of quaternized agricultural waste adsorbent for PFAS removal from real environmental matrix. 3.4. ATRP and amination for PFAS adsorption Aminated rice husk adsorbent was prepared via surface initiated ATRP and amination reaction by Deng et al. (2013). The derived adsorbent was utilized for the adsorption of PFAS, namely PFOS, PFOA and PFBA from aqueous solution. The ATRP initiators, bromoisobutyryl bromide (BIBB) were initiated onto the rice husk surface. This was followed by the grafting of polymer brushes of poly-glycidyl methacrylate (PGMA) via ATRP to produce the rice husk copolymer (RH-g-PGMA). An addition of tetrahydrofuran (2 mL) and ethylenediamine (2 mL) onto the derived RH-g-PGMA was performed in order to produce an aminated rice husk adsorbent. Initial PFAS concentrations investigated in the study were in the range of 0–0.5 mmol/L (below 400 mg/L). The adsorbent dosage utilized for batch equilibrium experiments during the adsorption process was 0.01 g of the aminated rice husk. Maximum adsorption capacities of 2.49, 1.70 and 2.65 mmol/g at pH 5.0 for PFOA, PFBA and PFOS, respectively, were reported in the study. It can be deduced from the FTIR spectra reported in the study that there was successful grafting of PGMA and amine groups on the rice husk surface. This is due to the presence of overlapping stretching vibrations of amine groups (that were absent on the original rice husk) on the derived aminated rice husk adsorbent. The XPS analysis performed in the study also verified the presence of grafted protonated amine groups and anionic groups (from the PFAS) on the surface of the modified rice husk material after the adsorption process. This is probably due to the electrostatic interaction between the anionic groups of PFAS and the proton rich amine groups of the aminated rice husk adsorbent. The preparation of a novel adsorbent–aminated rice husk was achieved in the study and its efficacy for adsorption of PFAS was also evident. However, the use of a good number of chemicals e.g. BIBB, PGMA, ethylenediamine and tetrahydrofuran for the synthesis of the aminated rice husk adsorbent is a major drawback to this technique. This is due to the tendency of these chemicals to constitute secondary wastes. The novel method for modifying agricultural waste materials for PFAS adsorption described in the study was not up-scaled for the removal of these pollutants from industrial effluents. 3.5. Burning and pyrolyzation for PFAS adsorption Chen et al. (2011) investigated the adsorption of PFOS from aqueous media using maize (Zea mays) straw-char, maize straw-ash and willow (Salix babylonica) tree sawdust-char, amidst other selected adsorbents. The maize straw was burnt on a stainless-steel plate in air (presence of oxygen) to derive the maize straw-ash whereas the maize straw-char and the willow tree sawdust-char were obtained by the pyrolyzation (heating in the absence of oxygen) at 400 ◦ C. The chars and ash in the study were characterized using BET. It was reported in the study that both the willow-derived char and the maize straw-char were negatively charged, with zero point of charges of 2.3 and 2.2, respectively. On the other hand, the maize straw-ash was found to be positively charged with a point of zero charge of 10.5. The initial PFOS concentration investigated in the study during adsorption kinetics experiments was 100 mg/L and the adsorbent dosage utilized was in the range of 5–30 mg (0.005–0.03 g). It was deduced from the study that it was difficult for PFOS to be adsorbed to the chars owing to electrostatic repulsion between PFOS and the chars as both are negatively charged (Brooke et al., 2004; Chen et al., 2011). Hence, low adsorption capacities below 170 mg/g were reported for both the maize straw-char and the willow—derived char. In contrast, the positively charged surface of the maize straw-ash enabled the occurrence of both hydrophobic and electrostatic interactions favourable for the adsorption of PFAS. Therefore, a high adsorption capacity of over 700 mg/g was reported for the maize straw-ash. In addition, the larger BET surface area, 38.3 m2 /g recorded for the maize straw-ash was also a key factor possibly contributing to the high adsorption rate. Though a high adsorption capacity was reported for the maize straw-ash, the feasibility of using maize straw-ash as an adsorbent on a larger scale should be taken into consideration. This is because the maize straw-ash was produced via

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burning. A reduction in product yield is usually observed with the use of burning and/or pyrolyzation techniques, as there is the tendency of the burning of some of the carbon content of the original agricultural waste material (Shinogi and Kanri, 2003). Therefore, a large quantity would be required to produce sufficient quantities of these materials for their application on a large scale, which may result in higher cost of energy. Such high cost of energy may nullify the cost-effectiveness that is supposed to come with the use of agricultural waste materials as precursors for adsorbent preparation. 4. Challenges The need to modify agricultural wastes, either by physical or chemical methods, due to inadequacies that may be inherent in raw agricultural wastes could pose financial challenges when considering the use of agricultural wastes for organic pollutants’ removal. Research has shown that inherent functional groups such as hydroxyls, carboxylates, amines, amides and others play a significant role in determining the adsorption capacities of different adsorbents. The task of determining which agricultural waste to utilize for the production of a suitable adsorbent, that is, the one(s) with the right components and/ functional groups in the appropriate fractions might be an uphill task. Thermal activation has been found to be one of the methods for treating agricultural wastes. However, for a vast majority of crop waste materials, e.g. maize tassel, rice husk, rice straw, corncob, sugar cane bagasse, pumpkin seed and others, thermal activation may not be a suitable procedure due to the crop waste materials’ natural make-up. They burn off easily in air between 160–200 ◦ C to produce soft ash, unlike wood which first goes through thermal degradation. The tendency to burn off easily during physical processing like thermal activation and varying burning rates are some of the challenges that come with the choice of using some crop waste materials for organics adsorption. Adsorption capacities of agricultural wastes could be affected by aromaticity and polarity and thereby by the structure of the components inherent in the agricultural waste (Xi and Chen, 2014). Lignin, hemicelluloses and cellulose have been noted as major storage media for organic pollutant, therefore, increased amounts of these in an agricultural waste material can contribute to higher adsorption rate for organic pollutants. Hence, crop waste materials with little or no cellulose and hemicellulose might not be perceived as promising candidates for the adsorption of organic pollutants in aqueous media. A gap in knowledge is the fact that most investigations, both in the recent past and currently, have focused on few specific groups of organic pollutants, usually dyes, phenolics and oils, with meagre foci on the removal of PFAS. A good number of assessments and evaluations have been performed only on simulated samples i.e. there is the lack of field trials. This is a gap that needs to be filled. There is need for more investigations on the application of promising agricultural-based adsorbents for research on real industrial effluents. 5. Perspectives for future research Despite the growing number of published data on the potential of agricultural wastes for the removal of organics, e.g., coconut shells and husks (Tan et al., 2008; Hameed et al., 2008), palm seed and pith (Rengaraj et al., 2002; Sathishkumar et al., 2007), sugar cane bagasse (Akl et al., 2014) and many others; none of these have been applied in the area of PFAS removal from aqueous media. There is still very scarce information on the removal of PFAS using adsorbents derived from agricultural wastes. A thorough review of published data on adsorption of organics using agricultural wastes was performed for the purpose of collating information for this paper, only a handful of studies (Deng et al., 2013, 2012, 2015; Fagbayigbo et al., 2017 amongst others) on PFAS were found. It is a case of either not much has been done or not much has been documented. There is need for more research on the use of agricultural wastes for the removal of PFAS from aqueous media. This need has led to the ongoing research by Omo-Okoro and co-workers where an agriculturally derived nanocomposite (maize tassel–silver nanoparticles) was successfully synthesized via microwave irradiation utilizing maize tassel powder and silver nitrate only. No reducing agents and stabilizing/binding agents were added. The irregular, spherical shape of silver nanoparticles in consonance with literature was confirmed on the newly formed nanocomposite material via EDX, SEM, TEM, FTIR and XRD. The aim of the ongoing research is centred on the synthesis of novel adsorbents and nanocomposite materials from agricultural wastes. The synthesized agro-nanocomposites will be applied for the adsorption of PFAS from PFAS contaminated effluents. 6. Conclusions

• Literature has shown agricultural wastes to be suitable precursor materials for preparing effective and suitable adsorbents for organic pollutants’ removal from aqueous media.

• Adsorbents have been successfully prepared via ATRP, quaternization, pyrolyzation activation and amination. • Most agricultural wastes are not suitable for adsorption in their raw forms and may need to undergo some forms of pre-treatments such as physical and chemical processes, namely: drying, autoclaving, grinding, milling, sieving and chemical modifications with reagents, in a bid to improve the biomaterials’ innate adsorption capacities. • Agricultural waste materials are modified to bring about enhanced performance via reduced particle sizes and wider surface areas. • When considering which agricultural-based material to utilize for commercial water treatment applications, some variables such as apparent density, their availability in powdered or granular form, regeneration capacity, the presence of impurities and market price need to be taken into account (San Miguel et al. 2006).

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• The interplay of operating conditions such as pH, contact time, initial adsorbent dose, stirring rate, reagent used for modification, temperature, ionic strength, interactions of competing organic and inorganic compounds in solution, analytical methods and techniques play a significant role in determining how effective an agricultural-based adsorbent is. Acknowledgements The authors are grateful to the Tshwane University of Technology, Arcadia, Pretoria, South Africa for the bursary provided to Mrs PN Omo-Okoro for her PhD programme and for providing an enabling environment that afforded the writing of this article. One of the authors, Mrs PN Omo-Okoro is grateful to Dr. CM Zvinowanda for the fruitful discussions and his useful inputs while writing this article. Conflict of interest The authors declare no financial or commercial conflict of interest. References Abdel-Halim, E., Al-Deyab, S.S., 2012. 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