Application of Agricultural Waste-Based Activated Carbon for Antibiotic Removal in Wastewaters: A Comprehensive Review

Abstract

Bisphenol A (BPA) is an industrial chemical used primarily in the manufacture of polycarbonate plastics and epoxy resins. BPA is considered an endocrine-disrupting chemical (EDC) because it interferes with hormonal systems. Over the decades, several techniques have been proposed for BPA removal in wastewaters. This study discusses recent advancements and progress of effective techniques for BPA removal, including membrane, adsorption, advanced oxidation process (AOPs), and biodegradation. The mechanisms of BPA adsorption on modified adsorbents include pore-filling, hydrophobic interactions, hydrogen bonding, and electrostatic interactions. Among the various agricultural waste adsorbents, Argan nut shell-microporous carbon (ANS@H20–120) exhibited the highest efficiency in removing BPA. Furthermore, the performance of magnetic treatment for activated carbon (AC) regeneration is introduced. According to the present study, researchers should prioritize agricultural waste-based adsorbents such as ACs, highly microporous carbons, nanoparticles, and polymers for the removal of BPA. In particular, the combination of adsorption and AOPs (advanced oxidations) is regarded as an efficient method for BPA removal. A series of relevant studies should be conducted at laboratory, pilot, and industrial scales for optimizing the application of agricultural waste-based AC to reduce BPA or other refractory pollutants from an aqueous environment.

1. Introduction

The production of chemical products, including disinfectants, insecticides, plastics, personal care items, detergents, and pharmaceuticals, is rapidly increasing to satisfy the requirements of industrial output [1]. During the production process, chemical wastes are produced, which can pollute food and water resources [2]. Along with the production process, the solid waste of industrial products has a large concentration of emerging pollutants, which can threaten human health [3]. These pollutants can have acute and chronic effects according to their chemical properties and target in the human body [4,5]. Although there are no strong international laws for the use of these pollutants in industrial products and their release into the environment, drinking water standards regarding their presence and concentration in aquatic environments must be observed in order to preserve human health [6]. Many of these pollutants can disrupt endocrine activity, including Bisphenol A (BPA), parabens, and phthalates [7].

BPA is a growing contaminant that is widely produced and used in a variety of industrial applications around the world. This pollutant is usually present in the structure of all kinds of plastics, canned foods, thermal paper, dental sealants and composites, and electronic products [8,9]. BPA was initially created in 1891 and has been widely produced since the 1960s when it was applied to the inside of plastics and cans [10]. BPA is primarily introduced into the environment through industrial effluent, leaching from plastic containers, and improper disposal of plastic objects. The persistent presence of BPA in aquatic ecosystems poses risks to both aquatic organisms and humans through the consumption of contaminated water and fish [11,12]. BPA exhibits estrogen-mimicking effects, leading to various health issues, including increased risk of hormone-related cancers, developmental abnormalities in children, and impaired reproductive health [11]. Additionally, BPA contamination in aquatic environments poses broader ecological concerns beyond human health, such as reproductive disorders (e.g., infertility, early puberty), metabolic disturbances (obesity and diabetes), cardiovascular diseases, breast and prostate cancers, and developmental or neurological impairments like cognitive deficits and behavioral changes. According to the mentioned concerns, it is necessary to find a promising method that can remove a significant concentration of this pollutant from water sources and guarantee human health in terms of the non-entry of this pollutant into the human body.

To remove BPA from water, a variety of remediation methods are employed. These methods include electron beam radiation [13], membrane catalytic system [14], photocatalysis [15], advanced oxidation process (AOPs) [16], and adsorption [17]. Among them, adsorption is particularly noteworthy due to its efficiency and ease of use, which will be discussed in detail below. It should be noted that adsorption not only effectively removes BPA but also improves water quality by removing other organic compounds and odors. Researchers have been looking for a material with high surface area, low cost and developed pore structure to effectively absorb BPA molecules from water and wastewater. Literature has shown that many absorbent materials are used for the removal of BPA, such as montmorillonite [18], zeolite imidazole framework [19], magnetic chitosan nanocomposite [20], N-doped porous biochar [21], polymer [22], magnetic biochar [23], natural zeolite [24], cellulose-incorporated imprinted materials [25], and activated carbon (AC) [2,26]. Among the mentioned adsorbents, the adsorbents that are synthesized from wastes have attracted researchers.

Utilizing agricultural by-products for environmental remediation and extracting value-added materials from such waste are key aspects of sustainable practices and effective waste conversion [27]. Agricultural waste conversion into biofuels [28,29], bio-organic fertilizer [30,31], and AC [32,33,34] presents a promising way to reduce dependency on natural resources and decrease the environmental impact of traditional farming practices. This shift highlights the vital role of waste utilization in producing eco-friendly products that not only promote waste reuse but also provide a key solution for addressing environmental pollutants [35,36]. For several decades, there has been a lot of discussion over the possibility of using agricultural waste (AW) as an alternative to the costly commercial ACs used in the treatment of wastewater that is polluted with BPA. If the amount of natural agricultural wastes was reduced and a greater proportion of those wastes were employed to clean up other pollutants, then the levels of pollution would drop. It was common practice in the past to employ commercial ACs (CACs) as adsorbents to remove BPA pollutants from wastewater.

Although existing reviews have broadly addressed adsorption-based removal methods for BPA [37,38,39,40,41], few have specifically explored agricultural waste-derived ACs [42,43]. This highlights a notable gap regarding the systematic assessment, comparative effectiveness, and scalability of these sustainable adsorbents. To bridge this gap, our review comprehensively examines the occurrence, health risks, and adsorption mechanisms of BPA, emphasizing agricultural wastes as promising, eco-friendly, and cost-effective adsorbents. In particular, we compare various agricultural waste materials, identify optimal preparation and activation conditions, and discuss technical and operational considerations relevant to real-world wastewater treatment scenarios. This study thus offers new insights and practical guidance for researchers and professionals working toward efficient and sustainable BPA remediation.

2. Characteristics, Sources, and Pathways Exposure, Detection, and Health Risk of Bisphenol A

Some new micro-contaminants, such as endocrine-disrupting chemicals (EDCs), threaten humans and animals because they interfere with the body’s natural hormones’ ability to complete their jobs. EDCs occupy hormone receptors and metabolic activities [44,45,46]. There has been a marked increase in the number of endocrine-disrupting substances detected in aquatic ecosystems in recent decades. One of the common endocrine disruptors found in water sources is bisphenol A. The rising demand for BPA in various commercial and industrial settings has elevated its presence in many water bodies, including wastewater, surface water, groundwater, and runoff. Polycarbonate and epoxy resins also benefit from BPA, which is a valuable precursor in their production [47]. BPA is a synthetic carbon-based material with the chemical formula of C₁₅H₁₆O₂ and a structure containing two 4-hydroxyphenyl groups, which imparts a mild phenolic odor.

Table 1. Chemical and physical properties of BPA.

Property	Index	Value
	Chemical Formula	C15H16O2
	Appearance	White crystalline solid
Physical property	Molecular weight	228.29 g/mol
	Heavy Atom Count	17
	Flash Point	227 °C
	Volatility	Non-volatile
	Odor	Mild phenolic odor
	Vapor Pressure	4.0 × 10−8 mmHg at 25 °C
	Boiling Point	220 °C at 4 mmHg
	Melting Point	153–156 °C
	Solubility in water	300 mg/L at 25 °C
	Chemical Structure
Chemical property	Partition Coefficient (LogP)	3.32
	Dissociation Constants	pKa = 9.6
	Chemical Classes	Endocrine Disruptors
	Structural Features	Two phenolic rings linked by isopropylidene bridge (propane bridge)
	Functional Groups	Phenolic hydroxyl groups (–OH)
	Chemical Class	Phenolic compound, Endocrine disruptor
	Reactivity	Reactive with strong bases, acids, oxidizers
	Stability	Stable under normal conditions; susceptible to oxidative degradation

provides the chemical and physical properties of BPA.

Table 1 presents the chemical structure of BPA, showing two phenolic rings joined by carbon–carbon bonds in the form of a bridge. The compound may possess antioxidant properties because the phenolic rings can donate hydrogen atoms to free radicals and stabilize them [48]. Moreover, the bridge linking the two rings can impact the compound’s overall stability, reactivity, and bioavailability [49]. BPA is commonly used as a cross-linker in the manufacturing of several plastic products and polycarbonate resin, including items such as plastic bottles, food and beverage lining, dental composite, etc. [50]. Demand for BPA production has grown continuously globally in recent years, and BPA consumption is expected to reach more than 10,000,000 tons by 2022 [51].

BPA is recognized as an endocrine disruptor and a harmful substance, leading to negative impacts on human health (such as issues with fetal brain development, cancer, diabetes, and obesity). Furthermore, its presence in aquatic environments could pose a significant ecological risk to the entire aquatic ecosystem [10]. BPA is also considered a top contaminant in wastewater treatment because of its high demand and toxic properties. Thus, removing BPA and its derivatives from water solutions is vital for environmental protection [52]. Recent research has revealed the harmful effects of BPA on human health. Human exposure to BPA has recently garnered attention due to its deleterious effects on male reproductive function [53] and disruption of thyroid function [54], and it has a link to metabolic syndrome, which encompasses obesity, insulin resistance, hypertension, and diabetes mellitus [55], hypertension, and cardiovascular diseases [56].

There are several ways of exposure to BPA, such as canned beverages and foods, dish and laundry detergents, polycarbonate water dispensers, tub and tile care products, vinyl fabrics, and thermal paper [57]. Empirical evidence indicates that the predominant vector for human exposure to bisphenols is via ingestion of foods and liquids tainted with BPA [58]. Some studies have shown that plastic containers can release BPA into food and water under normal use conditions [59,60,61]. However, BPA levels as high as 20 mg·g⁻¹ have also been reported on thermal receipt paper [62]. Indeed, during the utilization of products containing BPA, residual BPA can interact with water or cleaning agents that contain chlorine, leading to the synthesis of Chlorinated BPA (ClBPA) [63]. The employment of chlorine-based disinfectants in water treatment is crucial for eliminating harmful microorganisms before the water’s distribution to consumers [38]. Nonetheless, it has been observed that BPA may undergo reactions with chlorine compounds present in the water [64], This interaction can lead to the electrophilic aromatic substitution of chlorine atoms onto the phenolic rings of BPA, thereby generating chlorinated BPA derivatives at various positions on the molecule. Studies have reported that chlorinated BPA exhibits significantly higher estrogenic activity (about 10 to 40 times greater) compared to BPA alone [65], and low-dose experiments have shown that this increased activity can lead to the proliferation of breast cancer cells [66] and uterine endometrium cells [67]. The estrogenic activity of chlorinated BPA derivatives is thought to be higher than that of BPA itself [68]. Biological monitoring in humans serves as a quantitative analytical method to determine the concentration of BPA within the blood, urine, and tissue samples or to assess its metabolites, thereby facilitating a comprehensive assessment of exposure to BPA from all potential sources. The predominant metabolic pathway involves the conjugation of BPA with glucuronic acid in the liver, forming BPA-glucuronide. This primary metabolite, once synthesized, is rapidly absorbed by the gastrointestinal tract and subsequently excreted via the urine [69].

Figure 1 provides a comprehensive schematic representation of the sources, exposure pathways, toxicological profile, and consequent health implications of BPA. As delineated in the figure, BPA may infiltrate the human body via multiple vectors, including contaminated water, atmospheric deposition, dietary intake, consumer products, and environmental mediums such as soil or wastewater effluents, notably landfill leachate. Subsequent to environmental release, BPA exposure routes encompass dermal contact, respiratory inhalation, and dietary ingestion. The toxicodynamic profile of BPA is complex, with documented evidence of carcinogenic, reproductive, genotoxic, cytotoxic, and neurotoxic effects, in addition to its role as an endocrine disruptor and its estrogenic activity. These toxicological attributes of BPA contribute to a spectrum of deleterious health outcomes, such as obesity, reproductive pathologies, diabetes mellitus, oncogenesis, organ toxicity, neurodevelopmental anomalies, hypertension, and cardiovascular diseases. Figure 1 serves as an illustrative guide, synthesizing the critical aspects of BPA’s influence on human health and underscoring the imperative for regulatory scrutiny and environmental intervention.

Table 2. Laboratory methods of BPA detection.

Procedure	Matrix	Detection Limit	Ref.
fluorescence immunoassay	Unfiltered water	0.02 μg L	[75]
Surface Enhanced Raman spectroscopy (SERS)	water	0.1pg/mL	[76]
LC-MS	drinking water	7.0 ng/L	[77]
HPLC-diode array detection (DAD)	water and milk	0.07–0.16 ng/mL	[78]
GC-MS	bottled water and wastewater	0.30 ng/g	[79]
HPLC	pretreatment of effluent	10.5/μg/L	[80]
HPLC	urine	0.12 ng/mL	[81]
HPLC-tandem mass spectrometry (LC-MS/MS)	urine	0.4 ng/mL	[82]

delineates a comparative analysis of various laboratory methodologies employed to detect BPA. It systematically outlines the procedural steps, the analyte of interest, the type of matrix analyzed, and the detection limits attainable by each technique, thereby offering a lucid comparison of their sensitivity and applicability across a range of environmental matrices. The table enumerates several analytical approaches such as Liquid Chromatography (LC), Liquid Chromatography-Mass Spectrometry (LC-MS), Gas Chromatography-Mass Spectrometry (GC-MS), Fluorescence Spectrophotometry, and Fluorescence Immunoassay for the quantification of BPA. These methods are recognized for their high sensitivity in detecting BPA; however, they often necessitate intricate sample preparation, sophisticated instrumentation, and technically proficient personnel [72,73]. As indicated in Table 2, diverse methods have been implemented for the quantification of BPA in urine samples, including High-Performance Liquid Chromatography (HPLC), LC-MS/MS, and GC-MS. Notably, LC-MS/MS and GC-MS are distinguished for their superior sensitivity relative to other modalities, rendering them the preferred methods for BPA detection [74].

3. BPA Removal from Aquatic Environments

The treatment technologies of BPA in water and wastewater include chemical treatment, biological, and physical treatment. An overview of the methods is presented in Figure 2.

3.1. Chemical Treatment

Chemical treatment may be less necessary if BPA-free substitutes are found and used in the manufacturing process [83]. While these alternatives might not pose the same health hazards, their qualities are frequently comparable [84,85]. By reducing exposure, laws restricting the use of BPA in particular products can indirectly reduce the total amount of chemical treatment needed. Advanced oxidation procedures (AOPs) are frequently utilized to treat aqueous solutions polluted with BPA or other enduring contaminants [86]. During these processes, the contaminants are degraded by hydroxyl radicals (OH·, 2.8 V), sulfate radicals (SO₄⁻, 2.5–3.1 V), and other active species such as ozone (O₃, 2.07 V), superoxide anion radicals (O₂⁻·, 0.33 V), hydroperoxyl radicals (HO₂·, 1.7 V) into compounds with lower molecular masses upon interaction [87,88,89]. Various AOPs have been utilized to remove BPA from aqueous matrices, including ozonation [90], photocatalysis [91], Fenton oxidation [92] and UV light [93]. The ozonation process is considered suitable for decontaminating BPA in aquatic environments. Mutseyekwa et al. (2017) [94] examined the ozonation of BPA in acidic, neutral, and alkaline environments. Their findings showed that BPA decontamination transpired most swiftly in alkaline situations, necessitating 1.59 mg of ozone to eliminate each milligram of BPA within 30 min. Moreover, in a comparative study of different UV-based oxidation methods, the system that used the UV/K₂S₂O₈ process was more efficient for degrading BPA than UV/H₂O₂. The UV/K₂S₂O₈ process required a shorter time and achieved higher removal efficiency [95,96].

Hydrogen peroxide (H₂O₂) and iron salts (usually ferrous ions, Fe²⁺) are used in the Fenton oxidation process to produce hydroxyl radicals (•OH). These highly reactive radicals have the ability to efficiently degrade organic contaminants such as BPA [97]. Among the limitations of the Fenton oxidation process are the following: (1) Iron Hydroxide Sludge: The reaction results in the production of iron hydroxide sludge, which needs to be properly managed and disposed of; (2) Conditional Sensitivity: The effectiveness of the technique may be compromised if other substances in the wastewater consume hydroxyl radicals. (3) Toxic Byproducts: Additional processing may be required to eliminate potentially toxic byproducts produced during the oxidation process [98,99].

Photocatalysis, ultrasonic treatment, and various chemical methods are also highlighted in the literature as effective for BPA removal. However, these approaches often come with notable limitations, such as complex operation, high maintenance costs, the risk of secondary pollution [100], and difficulties in designing reactors that are both large-scale and highly efficient.

3.2. Biological Treatment

Biological treatment processes work by using biological agents, such as plants, enzymes and microorganisms, to biodegrade BPA into non-toxic substances. Biodegradation is considered the most efficient technique for removing BPA from aqueous solutions [101,102]. The literature presents several studies on the bioremediation of BPA. Kenro et al. achieved 100% degradation of 100 mg/L of BPA in river water under aerobic conditions over 14 days at 30 °C [103]. Ying et al. reported 50% degradation efficiency for BPA (initial concentration of 1 μg/g) in aquifer material under aerobic conditions over 70 days at 20 °C [104]. Dorn et al. achieved over 90% degradation efficiency for BPA (initial concentration of 3 mg/L) in chemical plant treated effluent under aerobic conditions over 4 days at 22–25 °C [105]. Also, Ogawa et al. used activated sludge (MLSS > 2750 mg/L) under both aerobic and anaerobic conditions to degrade BPA (initial concentration of 0.05 to 10 μg/L), achieving a degradation efficiency of 72–99% under aerobic conditions over 72 h at 25 °C, and an unspecified efficiency under anaerobic conditions over 30 days at 9–22 °C [106,107,108,109]. These studies highlight the influence of experimental conditions on BPA degradation efficiency, with generally higher efficiencies observed under aerobic conditions. It was reported that the removal efficiency of BPA in a biofilter-based reactor using aerobic granules can reach up to 93%. Microalgae was also used to remove BPA. In recent years, biotechnology processes based on phytoremediation have been applied effectively to eliminate BPA [110]. Utilizing plant endophytic bacteria offers a sustainable method for the removal of organic pollutants [111,112,113,114,115]. BPA biodegradation was shown to be more effective in activated sludge (100%) than in river water, according to the findings of the consortia, which were evaluated in both activated sludge and river water. The elimination of BPA through biological degradation has some limits, even though it offers indications of potential. The amount of dissolved oxygen that is present in wastewater is an important component that can affect the biodegradation process [116,117,118,119]. Current evidence suggests that BPA degradation is significantly more efficient under aerobic conditions, with limited or partial degradation occurring anaerobically [120,121,122,123,124]. Furthermore, biological treatment can result in BPA adsorption by biomass or other solids, potentially reducing overall removal efficiency [125,126,127,128]. Additionally, these processes require a controlled environment and long reaction times, suggesting that biological methods may not be as effective in large-scale wastewater treatment plants [101].

3.3. Physical Treatment

Physical treatment processes are well-known for their flexibility, simplicity, and high efficiency in removing various contaminants from aquatic solutions [129]. Various physical methods, such as adsorption [130], membrane filtration [131], and reverse osmosis [132], have been applied to remove BPA from contaminated water. Compared to other wastewater treatment technologies, adsorption has a low energy consumption and maintenance cost, high removal capability, and high retention arising from membrane fouling/clogging [133]. In conclusion, adsorption is widely regarded as the most effective and cost-efficient method for removing BPA from water and wastewater [134]. This environmentally friendly process is characterized by its simple design and ease of operation [100]. According to Ali, I., and Gupta, V.K., adsorption costs only 10 to 200 USD per million liters, but ion exchange, electrolysis, reverse osmosis, and electrodialysis cost between 10 and 450 USD per million liters [135]. Adsorption, a surface process, involves the transfer of adsorbate from a liquid or gas phase to the surface of an adsorbent, making it a popular method for removing inorganic, organic, and biological contaminants [136]. Each adsorbent has distinct physicochemical properties, allowing it to target specific pollutants for decontamination. The properties of the adsorbent typically affect the rate of adsorption [130].

As discussed, several technologies have been developed to remove EDCs like BPA from water efficiently. Advanced oxidation processes (AOPs) are key in breaking down BPA, utilizing methods such as ozonation, ultrasonic irradiation, dark and photo-oxidation, Fenton’s reaction, and electrochemical oxidation [137]. Moreover, the applications of membrane and biological technologies for the removal of BPA from aqueous solution have been reported. However, these methods appear less effective in dealing with large volumes of low-level pollutants, combining the generation of toxic by-product contaminants. Therefore, developing a cost-effective and eco-friendly method for industrial-scale BPA removal is necessary [137].

4. Adsorption

The process of adsorption involves moving an adsorbate from a liquid or vapor phase to the adsorbent’s surface or interface. This approach is universally applicable to the removal of emerging pollutants [138,139]. Diverse adsorbents with distinct physicochemical characteristics can be utilized to remediate various types of contaminants [140]. Conditions in the solution, the type of adsorbate, and the physicochemical characteristics of the adsorbent all have a role in determining the adsorption rate [130]. Environmental pollutants, adsorbate and adsorbent composition, temperature, and experimental parameters (such as contact time and solution pH) are among the variables that might influence the adsorption process [141]. Research into the adsorption isotherm, kinetics, and thermodynamics can shed light on the processes that regulate the loading of contaminants onto the adsorbent [1]. For a particular temperature and equilibrium, the amount of adsorbate that reaches the adsorbent’s surface or interface can be determined using the adsorption isotherm [142].

Adsorption processes are considered a promising method for removing BPA from aqueous solutions. Despite limitations such as the absence of a universal adsorbent and varying adsorption capacities across different materials, adsorption is recognized for its cost-effectiveness, energy efficiency, and ability to remove pollutants, even at trace concentrations completely. This makes it an attractive option for BPA removal. The adsorption mechanism is typically explained by examining the adsorbents’ physicochemical properties, the adsorbate’s characteristics, and data from adsorption studies [1]. Key factors influencing adsorption mechanisms include the surface morphology of the adsorbent and experimental physicochemical conditions [143]. Additionally, the effectiveness of the adsorption process is highly dependent on the careful design of the adsorbent [144]. Figure 3 shows that BPA adsorption on carbonaceous adsorbents occurs through many processes, including host–guest interactions, hydrophobic interactions, hydrogen bonding, electrostatic interactions, π–π stacking, n–π interactions, and pore diffusion effects [145].

As illustrated in Figure 3, the specific mechanisms involved in BPA adsorption onto modified carbonaceous adsorbents include: (a) Pore-filling mechanism, demonstrating the confinement of BPA molecules within the micropores of the adsorbents; (b) Hydrophobic mechanism, emphasizing non-polar interactions between BPA’s structure and the hydrophobic surface of the adsorbent; (c) Hydrogen bonding mechanism, showing the interaction between BPA’s hydroxyl groups and surface functional groups of the adsorbents; (d) Electrostatic mechanism, highlighting interactions between charged BPA molecules and oppositely charged adsorbent surfaces, influenced by solution pH and BPA’s dissociation.

Positive and negative charges are denoted by the plus and minus signs, respectively. Rovani et al. claim that electrostatic interactions are the primary process by which BPA adsorbs onto chemically produced nanoparticles of mesoporous silica derived from waste ash from sugarcane [130]. The study revealed that at solution pH levels below BPA’s pKa values, BPA adsorption onto MSN-CTAB was largely driven by hydrophobic interactions in the pH range of 9.6 to 11.3, with electrostatic interactions also playing a role. The anionic form of BPA was more strongly absorbed by the MSN-CTAB adsorbent than the undissociated form [146]. A hydrogen-bond-induced inclusion complex is formed when the cationic adsorbent interacts with the anionic adsorbate via electrostatic interactions while BPA and SB-β-CD are adsorbing to each other [1]. Through host–guest interactions, the hydroxyl groups (-OH) on the β-CD cavity edges operate as active sites for BPA adsorption. The adsorption efficiency of peanut-shell biochar for the removal of BPA was investigated by Wang et al. (2018), who came to the conclusion that the primary processes for BPA adsorption onto the peanut-shell biochar were hydrophobic effects, hydrogen bonding, and π–π electron donor/acceptor (EDA) interactions [147].

Biochar is produced through a process known as pyrolysis. During pyrolysis, biomass such as agricultural waste faces high temperatures without oxygen presence. During the pyrolysis process, the biomass is carbonized into a stable and carbon-rich material known as biochar. Biochar is the primary product during the two steps of AC production (carbonization and activation). Biochar has a wide variety of applications, especially for environmental remediation, such as water and wastewater treatment [148,149].

The adsorption efficiency of the adsorbents significantly relies on the preparation conditions. It was reported that increasing the pyrolysis temperature increases the hydrophobicity and the number of aromatic benzene rings in biochar. At higher pyrolysis temperatures, the biochar becomes a stronger π–electron acceptor, facilitating π–π EDA interactions with the π–electron donor BPA. Sun et al. made similar observations [150].

5. Removal of BPA from Aquatic Environments by Adsorbents

Up to now, diverse materials (either natural or engineered materials) have been inventoried providing new opportunities as adsorbents in the development of the adsorption capacity to treat BPA-contaminated aqueous solutions. The adsorbents have been classified as (a) natural materials such as clay, fullers earth, and diatomaceous earth; (b) modified/engineered natural materials such as activated alumina, and carbon; (c) synthetic materials such as zeolite, MOF, resin, and polymers; (d) bio-sorbents like chitosan, alginate, microorganism biomass and fungi; and (e) agricultural wastes such as rice husk, wheat barn, corn cob, bagasse, etc. Figure 4 represents the main classification of adsorbents [151].

AC has been the most widespread material for the elimination of BPA from water streams with various degrees of development from the lab- to pilot and full scale. It possesses a high specific surface area, good stability, and low surface polarity that makes it an excellent adsorbent for considerable adsorption of BPA. However, the main challenge to synthesizing AC from the raw materials (precursors) is its high cost [152]. Furthermore, the synthesis of AC is intricate due to its reliance on numerous physicochemical features. Nowadays, commercial ACs (CACs) appear to have poor BPA adsorption capacities and also require high energy and lots of chemicals for regeneration. The mentioned challenges have opened up exciting opportunities to develop low-cost and readily available carbonaceous materials like agricultural waste as feedstocks and design green adsorbents [153]. Due to many advantages such as multifarious functional groups, easily available, abundance in nature, chemical stability, renewable and biodegradable, and low cost, the production of ACs from agriculture waste materials has been widely utilized in removing BPA from aqueous solutions.

It has been known that AC has unique surface properties compared to other non-carbon adsorbents, such as clay. The surface of AC is generally non-polar or only slightly polar [125]. By the way, AC has been widely used to remove various pollutants.

Table 3. Characteristics of some synthetic adsorbents applied for BPA remediation.

Adsorbent	Surface Area (m2/g)	Micropore Volume	Mesopore Volume	(Total) Pore Volume (cm3/g)	Pore Size/Diameter (nm)	Reference
Pure Ca-montmorillonite	75.725	–	–	0.126	6.649	[154]
Hydrophobic Zeolite	504.5 ± 4.8	–	–	0.317 ± 0.006	–	[155]
Powdered AC (PAC)	1027	–	–	0.50	<3
CMK-3	1420	–	–	1.14	4.0	[156]
Soft templated carbon	476	–	–	0.49	7.0
Kaolinite/Fe3O4	7.62	–	–	0.029	15.04	[157]
Kaolinite/Fe3O4	65.15	–	–	0.207	12.70
Magnetic molecular polymers	142.90	–	–	0.158	4.41
MNIPs	150.80	–	–	0.177	4.68
Ca–Mt	54.44	–	–	0.139	10.19	[18]
Al–Mt	228.64	–	–	0.197	3.533
Zeolite synthesized from coal fly ash	91.50	–	–	–	–	[158]
W20	1777	–	–	–	–	[159]
W20N	1760	–	–	–	–
PAC	1326	–	0.9017	3.710	–	[160]
PAC-PNIPAM(1)	603	–	0.4665	3.835	–
PAC-PNIPAM(2)	313	–	0.2387	3.711	–
Silica@carbon	259	–	–	0.7	–	[161]
Hollow carbon porous nanospheres	477	–	–	1.1	–
Granular AC	896	–	–	–	–	[162]
Modified peat	0.66	–	–	–	–
Hydrophobic mesoporous material	1030	0.09	–	0.98	2.54	[163]
Organic–inorganic hybrid mesoporous material	750	0.42	–	0.48	0.96
Powdered AC	1780	0.59	–	1.59	–

shows the characteristics of some synthetic adsorbents used for BPA removal. This table comprehensively compares different adsorbents used for BPA removal, including their surface area, pore volume, and pore size. This table illustrates that an ideal adsorbent for BPA removal would have a large surface area, appropriate pore size and volume, hydrophobic surface, and functional groups that can interact with BPA molecules. However, the efficiency of BPA removal also depends on other factors, such as the pH of the solution, temperature, and the presence of other contaminants. Therefore, it is essential to consider all these factors when selecting or designing an adsorbent for BPA removal.

5.1. Natural Adsorbents

Clay has been widely utilized in multiple forms as an adsorbent to eliminate contaminants, such as BPA, from aqueous solutions [38]. Table 4 describes various natural adsorbents employed for the elimination of BPA. At 297 degrees Kelvin, the organoclay adsorbent had a maximum adsorption capacity of 151.52 mg/g. The researchers Wang et al. investigated the process of removing bisphenol A (BPA) from aquatic environments by employing organoclay that was generated from montmorillonite (MMT) in conjunction with several organic surfactants [164]. Zeolites are regarded as selective adsorbents; there are approximately forty natural zeolites and more than one hundred synthetic zeolites. Research conducted by Altmann and colleagues looked into the adsorption properties of BPA in aqueous solutions using hydrophobic zeolite at a temperature of 25 °C. The microporous zeolite had a significant affinity for BPA, with a capacity of 129.6 mg/g. This indicates that the adsorption mechanism was predominantly influenced by physical interactions, presumably via London dispersion forces [165]. Hexadecyltrimethylammonium (HDTMA) was added to zeolite that had been manufactured from coal fly ash (ZFA) and then examined for its ability to adsorb BPA [158]. It was discovered that the zeolites’ outside surfaces formed bilayer micelles of HDTMA. The surfactant-modified ZFA (SMZFA) had a far higher adsorption capacity for BPA than the unmodified ZFA, which had no affinity for the compound [158]. The two hydrophobic benzene rings of BPA positioned themselves toward the inside of the HDTMA bilayers, and it was discovered that the anions of BPA had a strong interaction with the positively charged heads of HDTMA. Likely, hydrophobic partitioning into the HDTMA bilayers and interaction between the oxygen atoms of BPA and the positively charged HDTMA heads were involved in the adsorption of uncharged BPA.

Chitosan, a natural polymer produced from seafood waste, has garnered interest as an effective bio-sorbent due to its cost-effectiveness relative to AC and its substantial concentration of amino and hydroxyl functional groups, providing significant adsorption capacity for aquatic contaminants. To remove BPA from aqueous solutions, a hybrid strategy that relied on chitosan and the oxidoreductase polyphenol oxidase (PPO) was utilized [166]. Optimal conditions for enzymatic quinone oxidation of BPA were found at pH 7.0 and 40 °C, with quinone chemisorbed onto chitosan beads, resulting in complete BPA removal within 4 to 7 h. Kimura et al. reported that chitosan in the form of porous beads was more effective for BPA removal than chitosan in solution or powder form. The removal efficiency of commercial chitosan (CC) and laboratory-synthesized chitosan (SC) was also evaluated. The optimal chitosan concentration was 0.06 g/L, with the highest BPA removal achieved using synthesized chitosan at pH 5, a contact time of 75 min, an adsorbent dose of 0.06 g/L, and a BPA concentration of 0.1 mg/L. The maximum adsorption capacities were 34.48 mg/g for synthesized chitosan and 27.02 mg/g for commercial chitosan [167,168].

Table 4. Characteristics of natural adsorbents for BPA removal.

Adsorbent	Surface Area (m2/g)	Reference
Peat	1.02	[169]
Sawdust	0.72
Bagasse	1.67
Rice husk	0.24

Table 4. Characteristics of natural adsorbents for BPA removal.

Adsorbent	Surface Area (m2/g)	Reference
Peat	1.02	[169]
Sawdust	0.72
Bagasse	1.67
Rice husk	0.24

5.2. Agricultural Waste Adsorbents

The three main structural components of lignocellulosic resources—cellulose, hemicellulose, and lignin—make up agricultural waste materials (AWMs) [170]. Mineral water (AWM) contains a number of inorganic elements in addition to oxygen (O), hydrogen (H), and carbon (C), including sodium (Na), calcium (Ca), magnesium (Mg), potassium (K), nitrogen (N), sulfur (S), phosphorus (P), and chlorine (Cl) [171]. The interaction between the organic and inorganic components of AWMs significantly impacts the adsorption process. Due to their effectiveness in pollutant removal, AWMs are widely used as adsorbents in adsorption science and technology.

Rice husks, corn stalks, and sugarcane bagasse are examples of agricultural waste that is abundant but usually underutilized [172]. Because of their porous structure and vast surface area, these materials have inherent adsorptive properties [173]. In addition to addressing disposal concerns, recycling waste into absorbents creates a valuable product that combats BPA contamination. Due to the high concentration of silica that facilitates the absorption of organic pollutants, this rice mill waste has great potential [174]. One study found that rice husks may successfully remove BPA from water solutions. Corn stalks rich in cellulose and hemicellulose have been modified to increase absorption capacity [175]. According to the literature, chemically treated corn stalks can significantly reduce BPA levels in wastewater [176]. The capacity of agricultural waste-based adsorbents to bind BPA improves with increasing lignin content, making it a prime choice for wastewater treatment [177]. There are several ways that BPA may adsorb onto agricultural waste, including chemical adsorption (covalent bonding) and physical adsorption (van der Waals forces) [175].

5.3. Activated Carbon (AC)

A popular adsorbent for air and water filtration, AC was first used to remove contaminants, including color, taste, and smell, from wastewater and industrial runoff [178]. Ingredients like wood, lignite, coal, or even coconut shells go through an activation procedure to become this. Carbon materials can be chemically activated by adding inorganic salts that dehydrate them or thermally activated by coking and combustion. There are inhomogeneous layers of microcrystalline graphite on the inside of the material, and its ash content is less than 15% after activation [179].

By encouraging the development of functional groups containing oxygen, the surface can be further altered by thermal treatment, ammonization, acid treatment, or oxidation [180]. Surface acidity is a result of carboxylic, phenolic, or carbonyl groups, in contrast to the basic nature of chromene and pyrone-type structures [181]. Then, interactions with these surface groups will result in the adsorption of micropollutants [182]. During the adsorption process, electrostatic and hydrophobic interactions were regarded as the main adsorption mechanisms [183]. Adsorption is also affected by non-specific factors like van der Waals forces [184]. AC has a very large inner surface area (800–1800 m²/g), which is its most distinctive feature [184]. A material’s adsorption capacity is directly proportional to its internal surface area, as is generally recognized [185]. By measuring the volume of physical gas adsorption, the Brunauer, Emmett, and Teller (BET) surface area is determined, making it the principal metric used to describe the adsorption capacity of AC. But this method does not tell you anything about the surface structure, which is made up of micropores smaller than 2 nm, mesopores between 2 and 50 nm, and macropores larger than 50 nm [185]. AC’s adsorption behavior is affected by both its internal surface area and the distribution of its pore sizes, according to multiple experiments. To adsorb micropollutants, AC with a dense network of micropores is optimal [186]. There are a number of material properties, including hydrophobicity, surface charge, and wettability, that affect AC adsorption [183]. In this context, AC is frequently described by referring to its ability to adsorb particular indicator molecules, such as iodine, methylene blue, or nitrobenzene, that are present in the environment [187]. The chemical characteristics of the adsorbates and AC features both impact the adsorption effectiveness. Due to the presence of easily adsorbable ionized species, hydrophilic compounds were shown to be adsorbed to a greater extent than hydrophobic compounds, according to multiple investigations [188]. Thus, the extent of adsorption can be effectively correlated with the octanol–water partition coefficient of the adsorbate [189]. Other factors that were reported to promote the adsorption capacity of ACs are positive charges [182], high aromaticity [190], low polarity [184] and low molecular mass [182].

Municipal wastewater may possess a greater quantity of organic matter, including humic and fulvic chemicals, in comparison to industrial wastewater or drinking water. The presence of various elements in the wastewater matrix can diminish the efficacy of AC in eliminating micropollutants [191]. Adsorption competition is especially important if there are a lot of neutral organic components and low molecular weight acids in the wastewater matrix [192]. Prior research has examined in detail the possibility of AC adsorbing pharmaceutical residues and other micropollutants from wastewater.

Activation of char produced during the carbonization process in kilns is achieved through physical methods such as steam activation, O₂, and CO₂, or through chemical methods involving impregnation with acids like HNO₃, H₂SO₄, and H₃PO₄, alkaline agents such as NaOH, KOH, and NH₃, and salt treatments with K₂CO₃, ZnCl₂, and KHCO₃. The second stage of the process—high-temperature pyrolysis—is essential following chemical activation to generate active adsorption sites and eliminate contaminants. The adsorptive properties of AC can be enhanced during or subsequent to the carbonization phase by subjecting the char to gases or chemicals earlier in the process [89]. Processing the precursor at moderate to high temperatures changes it by reducing its solid mass and creating pores at the same time.

The transformation of utilized AC into biological AC, also known as BAC, occurs when the carbon becomes saturated with organic molecules following its use. Microorganisms feed on the organic compounds that are adsorbed onto AC, which results in the formation of a biofilm both on the surface and within the pores of the carbon. The generation of BAC is facilitated by this mechanism. It has been demonstrated that BAC is capable of effectively absorbing a wide range of contaminants, including organics, inorganics, heavy metals, and BPA. Eliminating pharmaceutically active compounds was accomplished by Sbardella and colleagues by the utilization of the BAC-filtration method [193]. Other researchers have also used BAC systems to adsorb pesticides [194] and BPA [195].

Table 5. Previous research in the removal of BPA using ACs and other carbon-based materials.

Adsorbents	Qmax (mg/g)	Ref.
Sludge-Based ACs	285.8	[196]
Activated sunflower stem biochar	365.81	[149]
N-doped porous carbon-based	462.5	[197]
Activated seaweed pyrocarbon with a melamine (TSWP–M)	270.5	[198]
AC from coffee residue	105	[199]
Walnut shell AC (AC-Ws)	238.63	[200]
AC produced from waste coffee grounds	123.22	[201]
AC with shrimp shell-based precursor	207.77	[202]
AC derived from cores of nuts of Sapindus mukorossi (CNSM–ACH)	216.99	[203]
Powder AC (PAC) with manganese oxide (MgO)	9.200	[204]
Doum (Chamaerops humilis) fiber	56.11	[205]
Commercial AC (F400)	407.00	[206]
AC from Kraft lignin	220.000
Carbon xerogel	78.000
Calcium alginate/organo-activated bentonite	252.900	[207]
Calcium alginate/AC	419.000

. Presents the maximum adsorption capacity (Qmax) of some carbon-based adsorbents for BPA.

5.3.1. Physical Activation

The carbonization process, also known as pyrolysis, is carried out in an oxygen-free environment. This is followed by activation in an oxidizing gaseous environment (such as steam, CO₂, and N₂ or air) at elevated temperatures ranging from 800 to 1100 degrees Celsius. The physical activation of AC is a two-stage process. The production of AC, which is characterized by a porous structure, is the outcome of the phenomenon known as physical activation. This method is considered ecologically friendly due to the absence of chemicals and additives, and it is also rather cost-effective. Activity that requires physical exertion requires a significant investment of both time and energy. A limited adsorption capacity is displayed by the AC that has been manufactured [208]. The carbonization process entails the pyrolytic heat treatment of raw materials under diverse atmospheric conditions at high temperatures. A porous carbon structure is formed through the elimination of volatile constituents and gases (CO, H₂, CO₂, and CH₄), facilitating subsequent activation processes [209].

In order to enhance the fundamental features of the carbonized biomass (biochar), such as pore distribution, specific surface area, thermal–mechanical stability, and porous volume/density, the activation procedure is required to generate AC. To begin the process of developing the microporous structure, aromatic groups are subjected to oxidizing agents during the first phase. Following this, the second phase of the process is characterized by the disintegration of the walls of pores, which results in the enhancement of the pore structure and the generation of big pores. The process of activation can be broken down into two categories: physical and chemical [210]. It should be noted that due to the synergistic effects of pore formation, pore expansion, pore combination, and pore collapse, the porous structure of AC is developed [211].

The steam activation process is a physical approach that can be used to activate air conditioning. There is a substantial relationship between the parameters of steam activation, which include activation temperature, activation time, and steam flow rate, and the surface area and porosity of activated biochar. There was a correlation between the activation temperature and an increase in both the surface area and the pore volume [212]. During the initial activation phase, the formation of new pores occurs with increasing activation times, leading to an increase in surface area. As the activation process continues, existing pores primarily expand, with minimal new pore formation, resulting in decreases in surface area and pore volume [213].

Mesoporous and microporous structures can be formed when biochar is physically activated by gas, which increases its pore volume and surface area [214]. Various gases, including CO₂, N₂, NH₃, air, O₂, or their mixtures, are frequently employed in the activation process. Carbon dioxide is the most frequently utilized activation gas. The reaction occurs directly with the char as described by the Boudouard reaction (C(S) + CO₂ = 2CO) [215]. The Boudouard reaction is thermodynamically favorable at temperatures over 710 °C [215]. Hydrochars made from hickory and peanut hulls were studied by Fang et al. (2016) [216]. They looked at how the physicochemical properties changed depending on the activation temperature of CO₂ (600–900 °C) and the duration (1 or 2 h). The surface area and pore volume of activated biochar were shown to grow with increasing activation time and temperature, according to the results. In addition, the pore structure and surface properties of biochar could be enhanced by a combination of CO₂ and other gases. The biochar that was physically activated by heating maize stalks with CO₂ at 850 °C showed a microporous structure and a specific surface area of up to 880 m²·g⁻¹ [217]. Cotton stalk biochar activated by CO₂/ammonia at 500–900 °C indicated that this method combined the advantages of CO₂ and ammonia activation resulting in a high surface area up to 627.15 m²·g⁻¹, accompanying the introduction of N-containing groups into biochar [212].

5.3.2. Chemical Activation

Chemical activation has been known as a method of using chemical compounds as an activating agent during the second step in the activation process for AC production. These chemical compounds have been classified into four main groups including alkaline or basic (KOH, NaOH), acidic (HNO₃, HCL, H₃PO₄ and H₂SO₄), neutral (ZnCl₂, K₂CO₃), and self-activating agents [211,218]. The major advantages of chemical activation methods which make it a preferred method compared to physical activation are requiring low temperatures, low cost and simple production procedure, high surface area and superbly developed micropores [218].

After oxidation, functional groups (such as carboxylic acid or amine) and molecules (like cyclodextrin) are grafted onto the surface of AC using chemical, electrochemical, plasma, or microwave methods. Oxidation can be attained by electrochemical oxidation [219], air oxidation [220], chemical modification [221], and plasma treatment [222] or ozone treatment [223]. In the next sections, a comprehensive explanation of a number of significant chemical activation techniques that have been utilized for the surface modification of AC throughout the course of the past few decades has been provided.

Acid Activation

Acid activation enhances the porous carbon surface, improving its acidic properties, hydrophilicity, and contaminant adsorption capacity. This process involves impregnation with acids such as H₂SO₄, H₃PO₄, HNO₃, and HClO₄, with nitric and sulfuric acids being the most commonly used activators [224]. For example, Aggarwal et al. utilized nitric acid, ammonium persulfate, and hydrogen peroxide for oxidation, followed by gaseous oxygen treatment at 350 °C and degassing between 400 °C and 950 °C to improve the carbon–oxygen surface chemical structures [225]. Acid treatment effectively alters surface chemistry, enhancing the adsorption of contaminants. Among these acids, H₃PO₄ is particularly notable for achieving a high surface area (907–1033 m²/g) during activation [211].

Alkaline Activation

Alkaline activating agents are categorized by their dissociation potential in aqueous solutions, producing hydroxide ions (OH⁻). These are classified into three types: strong alkaline agents like KOH and NaOH, moderately strong agents such as K₂CO₃ and Na₂CO₃, and weak agents including K₂SiO₃, Na₂SiO₃, K₂B₄O₇, Na₂Al₂O₄, and K₃PO₄ [226]. While alkaline activation does not encourage meso- and micropore development, it typically results in narrow and wide microporosity [227].

Alkaline treatment of AC enhances its surface charge, promoting higher adsorption of negatively charged species. Treating AC under inert atmospheres, hydrogen, or ammonia at elevated temperatures is highly efficient for producing porous carbons with essential surface characteristics [228]. Specifically, treating AC with NH₃ at 400–900 °C forms basic nitrogen functionalities such as amides, aromatic amines, and protonated amides. Basicity is further enhanced by pyridine-like structures at temperatures above 600 °C [229,230]. These functionalities facilitate stronger dipole–dipole interactions, hydrogen bonding, and covalent bonding, improving the adsorption of organic compounds like phenol. Moreover, modifications like nitric acid treatment, pyrolysis with urea impregnation, and partial oxygen gasification generate basic groups and carbonyls on the AC surface [211]. Among alkaline agents, potassium hydroxide (KOH) stands out as the most efficient, producing activated carbon with a high specific surface area [226]. During activation, reactions such as the interaction of carbon with K₂O lead to an increase in BET surface area, optimizing AC’s adsorption properties. This efficiency makes KOH a widely used agent for synthesizing superior activated carbons [208,231].

2KOH → K₂O + H₂

(1)

C + H₂O → H₂ + CO

(2)

CO + H₂O → H₂ + CO₂

(3)

K₂O + CO₂ → K₂CO₃

(4)

K₂O + H₂ → 2K + H₂O

(5)

K₂O + C → 2K + CO

(6)

Impregnation

Impregnation is a crucial process for enhancing the adsorption capacity of AC by binding chemicals or metal particles uniformly to its porous surface. This synergy between the impregnated substances and the AC structure significantly improves its functionality [232].

Metals such as silver [233], copper [234], aluminum [235] and iron [236] are widely used for impregnation, effectively enhancing the adsorption of pollutants like heavy metals, BPA, fluoride, and cyanide in water solutions.

Iron salts are particularly notable in improving AC performance. Dastgheib et al. demonstrated a tenfold increase in arsenic adsorption by treating AC with iron salts, which facilitated ferrous ion adsorption and the formation of arsenate complexes. Follow-up studies revealed that iron impregnation, combined with high-temperature ammonia treatment, boosted dissolved organic matter (DOM) absorption by 50–120%. Additionally, Ghorishi et al. (2002) found that chlorine impregnation amplified BPA adsorption in fixed-bed systems by 2–3 times [237]. For arsenic removal, Chang et al. (2010) developed a multi-step impregnation process using ferrous chloride [238]. This method yielded stable crystalline and amorphous iron nanoparticles embedded within granular activated carbon (Fe-GAC). Sodium hydroxide treatment further enhanced the stability of these materials, even under typical water treatment pH conditions [239]. Isotherm tests and rapid small-scale column tests (RSSCTs) indicated significant arsenic adsorption, with internal iron-loading increasing to 9–17% and combined loading reaching up to 33.6% [240].

Microwave Activation

Microwave activation has gained significant attention for modifying AC due to its ability to facilitate uniform and rapid thermal reactions at the molecular level [241]. Unlike traditional heating methods, microwave heating transfers heat from the interior to the exterior of the material, eliminating the need for fluid-based heat transfer, and enabling quick and efficient heating. The process involves no direct contact between the microwave source and the material being treated, which simplifies management and reduces contamination risks. Microwave heating offers additional benefits such as high-temperature capabilities, energy efficiency, improved chemical reactivity, compact and lightweight equipment, ease of maintenance, and affordability [242]. Adhoum et al. demonstrated the effectiveness of microwave activation by using a nitrogen atmosphere to produce AC from modified bamboo [243]. During the modification process, surface acidic groups decreased while surface basicity slightly increased, resulting in an increased point of zero charge (pHpzc). Additionally, micropores expanded into larger structures, enhancing pore volume and average pore diameter, which improved adsorption efficiency. The study identified micropore enlargement as the key mechanism behind the improved Bisphenol A (BPA) adsorption on modified AC. Adsorption isotherm analyses revealed that Freundlich’s model was more applicable for the modified carbons, while Langmuir’s model suited the virgin carbons. Further, researchers have explored the use of microwave irradiation for activating other precursors, such as corncob furfural waste, with zinc chloride as a chemical activating agent [244]. These studies highlight the versatility of microwave-induced treatments in altering the surface chemistry of AC, enabling enhanced adsorption capacities for various water and wastewater pollutants. Foo and Hameed have extensively focused on developing microwave-assisted activated carbons, further solidifying microwave activation as a promising method for sustainable water treatment technologies [245].

Ozone Activation

Ozone activation is a notable method for modifying AC and enhancing its ability to remove harmful chemicals from water. Ozone is widely used due to its strong oxidizing properties and environmental compatibility. Recent advancements have explored the use of air cooling in conjunction with ozone, which eliminates the need for traditional ozonation setups. This combined system has shown enhanced adsorption efficiency, initially attributed to AC’s inherent adsorption capacity [246,247]. However, subsequent studies revealed additional mechanisms at play, including the reaction of ozone with organic matter adsorbed on AC and the generation of free radicals when dissolved ozone interacts with AC. These radicals contribute significantly to the mineralization of organic matter. Research by Rivera-Utrilla and Sanchez-Polo demonstrated the effects of ozone treatment on the surface characteristics and adsorption behavior of commercial AC (Filtrasorb 400) with respect to naphthalenesulfonic acids [248]. The experimental findings showed that the adsorption capability significantly decreased as the number of sulphonic groups in the aromatic ring increased.

Plasma Activation

Plasma activation involves exposing AC to plasma under vacuum or atmospheric pressure, with air or oxygen used to generate the plasma. While the structural changes in AC are minimal after plasma oxidation, the surface chemistry undergoes significant alterations due to the aggressive reaction of oxygen free radicals with carbon atoms at the edges of graphene layers, enhancing surface acidity. Dielectric barrier discharge (DBD) plasma has been used effectively to treat granular AC, improving its adsorbability for metal ions [249]. Using helium-based plasma, oxygen radicals are generated by mixing helium and oxygen as feeding gases. This process facilitates metal ion adsorption through ion exchange between surface hydrogen ions and metal cations in solution. The surface oxidation of AC by helium-oxygen DBD plasma results in an increased quantity of hydrogen ions released from functional groups, promoting weakly acidic functional group formation, essential for metal cation adsorption. Experiments revealed that plasma-treated AC achieved nearly 3.8 times higher adsorption capacity for iron cations (Fe²⁺) compared to untreated AC when treated for 30 min in a helium DBD reactor with 4% oxygen content [250]. Material investigations (using FTIR, XPS, and BET analysis) attribute this enhanced adsorbability to changes in surface chemical structure rather than physical structure. Oxygen plasma treatment of AC fibers (ACFs) demonstrated the introduction of oxygen-containing functional groups, such as C6H5OH and O=C=O, although it resulted in a slight reduction in specific surface area. Plasma-treated ACFs achieved higher HCl removal efficiency compared to untreated samples. Omer et al. further concluded that plasma treatment with high oxygen concentration and well-developed micropores enhanced adsorption performance [193]. While plasma activation requires initial investment in equipment and consistent power to generate plasma, it is regarded as an effective and cost-efficient method for modifying AC and removing hazardous substances like HCl vapors. These findings underscore the potential of plasma treatment in enhancing AC’s adsorption capabilities and enabling advanced applications in environmental remediation

5.3.3. Biological Activation

As AC that has been used up becomes saturated with organic molecules, a new type of carbon called biologically AC (BAC) is created. Bacteria feed on the organic material in the pores of AC, creating a biofilm both outside and inside the pores. This process yields AC that is biologically active. BAC may efficiently adsorb a wide range of contaminants, including organics, inorganics, heavy metals, and EDCs. We used BAC to remove EDC, namely 17 β-estradiol, from drinkable water. BAC was more effective than GAC at removing 17 β-estradiol, with an effluent concentration of 50 mg·L⁻¹ [164]. According to a column investigation, steam and chemically activated AC produced 64.5% and 81% DOC elimination, respectively. At several points throughout the treatment process, especially in filtration units, BAC is used in combination with other methods [166]. To extract medicinally useful substances, Sbardella et al. (2018) used BAC filtration [193]. The following drugs were removed with varying degrees of efficiency by the BAC-filtration process: azithromycin (63%), bezafibrate (67%), ofloxacin (77%), irbesartan (79%), propranolol (83%), and ciprofloxacin (86%). Adsorption of pesticides and phenols has also been accomplished by other researchers using BAC systems [212]. Figure 5. Presents the main pathways used for synthesizing AC.

As illustrated in Figure 5, the synthesis of activated carbon involves three primary pathways: physical activation, chemical activation, and biological activation, each tailored to achieve distinct material properties. Physical activation entails carbonizing raw materials, such as biomass, in an oxygen-free environment at high temperatures (400–850 °C), followed by an activation phase using oxidizing gases like steam or CO₂ at temperatures up to 1100 °C. This method develops a porous structure ideal for adsorption. Chemical activation uses chemical agents like phosphoric acid (H₃PO₄) or potassium hydroxide (KOH) during carbonization at moderate temperatures (400–700 °C), which enhances porosity and surface area while requiring lower energy than physical methods. Lastly, biological activation utilizes microorganisms, such as bacteria or fungi, to degrade biomass precursors and generate porosity. This eco-friendly approach is often coupled with low-temperature carbonization to improve the material’s stability and adsorption capacity. Each pathway contributes unique advantages, catering to specific applications in water purification, gas adsorption, and environmental remediation. Activated carbon can be engineered for superior efficiency and sustainability by optimizing these processes.

5.4. Integrated Technologies

Independent application of specific BPA treatment methods may face various removal challenges. Various techniques have been combined to improve the efficiency of BPA removal and facilitate mineralization. The combination of ozonation and UV-photolysis improves the efficiency of bisphenol A (BPA) removal. The mineralization process may range from 80% to 100% [251]. The removal of BPA from drinking water was investigated by Kim et al. (2008) [252]. They used a nanofiltration approach in conjunction with homogeneous catalytic oxidation, and they found that 90% of the BPA was destroyed within 3 min. The feasibility of using an adsorption technique augmented by biodegradation to eliminate BPA and other EDCs was studied by Castellana and Loffredo (2014) [253]. With a maximum removal effectiveness of 99% for BPA, the treatment method that utilized a blend of fungi (Trametes versicolor), green compost, and coconut fiber was successful. Zhang et al. (2011) successfully removed BPA through a hybrid method that integrates microfiltration and oxidation with β-MnO₂ nanowires [254]. Considerable advancements have been made in employing integrated methods for BPA removal; however, numerous technologies still exhibit limitations.

6. Future Perspectives

Future research efforts should prioritize the optimization and standardization of methods for synthesizing and activating agricultural waste-based ACs. This includes detailed investigations into the impacts of varying preparation conditions such as temperature, activation agents, and duration on the performance and economic feasibility of adsorbents. Additionally, it is crucial to conduct extensive comparative studies among different agricultural waste-derived adsorbents to identify optimal materials for specific wastewater treatment applications. Comprehensive environmental life-cycle assessments (LCAs) and economic analyses are necessary to support the real-world implementation of these sustainable adsorbents. These analyses should encompass energy consumption, environmental impacts, regeneration costs, and overall treatment economics, ensuring both environmental sustainability and economic practicality. Research should also explore effective and environmentally friendly regeneration techniques, such as magnetic treatment or chemical regeneration, alongside methods for the safe disposal or reuse of spent adsorbents. Developing integrated treatment technologies that combine adsorption with advanced oxidation processes, biodegradation, or membrane filtration could significantly enhance BPA removal efficiency. Lastly, pilot-scale and industrial-scale studies are required to validate laboratory-scale findings, address scalability challenges, and provide reliable data to facilitate commercial adoption. Such research initiatives will advance the understanding and practicality of using agricultural waste-derived ACs, ensuring effective management and sustainable remediation of BPA-contaminated water resources.

7. Conclusions

This review highlights the critical importance and effectiveness of agricultural waste-based ACs for the removal of BPA from wastewater. BPA, a widely utilized chemical in the manufacturing of plastic and resin products, poses significant risks due to its endocrine-disrupting capabilities, leading to severe health impacts, including reproductive disorders, metabolic diseases, cardiovascular problems, cancer risks, and developmental issues. Its presence in water systems also negatively affects ecological health, impacting wildlife and aquatic organisms. Agricultural waste materials have emerged as promising alternatives to commercial ACs, primarily due to their cost-effectiveness, wide availability, sustainability, and considerable adsorptive capacities. Through various activation techniques, including physical, chemical, microwave, ozone, plasma, and biological methods, agricultural wastes such as rice husks, corn stalks, bagasse, and nut shells have demonstrated substantial potential for efficiently removing BPA from contaminated waters. In particular, Argan nut shell-derived microporous carbon has shown outstanding adsorption efficiency, positioning itself as a viable candidate for practical applications in wastewater treatment. The adsorption process primarily involves mechanisms such as pore-filling, hydrophobic interactions, hydrogen bonding, and electrostatic interactions. Understanding these mechanisms further enhances the selection and optimization of appropriate adsorbent materials. Overall, the adoption of agricultural waste-derived ACs aligns with circular economy principles, promoting waste valorization and offering a sustainable solution to the critical environmental challenge posed by BPA contamination.