Low-Cost Adsorbents for the Removal of Pharmaceuticals from Surface Waters

Abstract

For decades, there has been increasing interest in pharmaceuticals’ prevalence in water bodies. This represents a major challenge in providing clean water, free from pharmaceutical contaminants, in different parts of the world. The misuse and overuse of pharmaceuticals, their elevated concentrations in surface waters, and their negative impacts on humans, aquatic organisms, and ecosystems cannot be ignored. Significant efforts have been made toward the discovery of efficient water treatment techniques. Various technologies have been researched and applied, including co-precipitation, membrane separation, ion-exchange, oxidation, adsorption, and biochemical processes. Amidst these technologies, adsorption is considered a promising due to its unique advantages. This review discusses the pharmaceuticals that have been detected in surface waters in concentrations ranging from ng/L to μg/L. It also offers insights into the diverse applications of low-cost adsorbents to deal with pharmaceutical water pollutants and various parameters influencing the adsorption process. This article will assist in promoting the utilization of sustainable, low-cost adsorbents with high adsorption efficiencies in the water treatment process, and it will aid environmentalists in devising strategies for anticipated challenges and provide policymakers with valuable guidance.

1. Introduction

The increasing world population and the increase in life expectancy coupled with modernization have resulted in a growing consumption of pharmaceuticals. Recent medical advancements have made it very easy to access pharmaceuticals for veterinary care, industrial applications, health care, food preservatives, and agricultural needs [1]. Pharmaceutically active constituents such as antibiotics, beta-blockers, anti-inflammatories analgesics, steroidal hormones, X-ray contrast media, and lipid regulators have been utilized globally to meet health care needs in both animals and humans. The use of these pharmaceuticals is expected to double in the coming years due to the increasing age of the population and advancements in health care standards [2]. They might be solely present or mixed, which might present synergistic or additive effects with various contaminants [3]. Pharmaceuticals find their way into the aquatic environments via various routes, which include animal waste, landfill leachates, indiscriminate waste disposal, domestic sewage effluents, hospital waste, and drain water, as shown in Figure 1 [4]. Pharmaceuticals have been detected in the aquatic environments in many countries such as Canada, Italy, UK, USA, South Africa, China, Brazil, and Germany [4]. Their concentrations in water bodies such as surface water, sea water, ground water, and wastewater range between 10 and 100 μg/L, and their effects could be toxic and diverse due to long-term exposures [2]. The toxic effects of these contaminants on human health and aquatic organisms include endocrine conduct disturbances, reproductive impairments, cancer, dysfunctional gene expression, organ damage, and the development of resistant bacteria [5]. Pharmaceuticals exhibit characteristics that include thermal/chemical stability and hydrophilicity, which contribute to their non-biodegradable nature in aquatic media [6].

Research has evolved from raw low-cost adsorbents toward engineered low-cost materials that are affordable while improving on selectivity, recovery, and kinetics. Some notable adsorbents, such as composite and magnetic biochar, facilitate rapid separation and enhanced surface chemistry [2]. Electro-assisted adsorption has been reported to enhance mass transfer and facilitate in situ regeneration in flow or fixed-bed processes [7]. Machine learning frameworks have been used to predict the capacity of an adsorbent–pharmaceutical pair [6]. Also, several studies have reported adsorbent performance under realistic conditions and in fixed-bed columns for scale-up purposes [8]. These trends involve a transition from batch removal to a regenerable, robust, and model-informed adsorption system for surface water treatment.

Wastewater treatment plants (WWTPs) are the main source of pharmaceuticals due to emerging pollutants from domestic, agricultural, industrial, and municipal effluents, which become concentrated at WWTPs for the purpose of treatment [2]. Conventional wastewater treatment has been reported to be ineffective in the removal of pharmaceuticals due to their solubility, and their initial design is not meant for the complete removal of contaminants present at relatively low concentrations [9]. The main purpose of WWTPs is the separation of large particles, organic compounds, and nutrients present in water at concentrations of mg/L and g/L. Due to ineffectiveness of WWTPs, these pharmaceuticals are released into the aquatic environment as complex compounds in concentrations different from the initial concentrations in WWTP influents [10]. The authors of publications [8] reported that only a few contaminants are found in lower concentrations in WWTP influents compared to the relatively high concentrations in WWTP effluents. This can be appropriated to several chemical combinations due to the formation of metabolites and compounds that are more toxic than the original ones [11]. Recent advances in the methodological processes of contaminant fate and transport modelling have equipped us with invaluable tools for the prediction of pharmaceutical persistence and mobility in aquatic systems, which include the non-equilibrium transport modelling framework, as described by [12]. Comparably, complementary treatment approaches such as advanced oxidation processes represent a promising method for the removal of complex contaminant mixtures, as illustrated in a pesticide-laden water study by [13].

Several advanced treatment methods have been reported to be efficient in the removal of pharmaceuticals from water, which include membrane filtration [10], advanced oxidation techniques [11], photocatalysis [14], biological methods [7], and electrochemical methods [14]. Some of the challenges associated with these methods include the high costs, vigorous operational procedures, longer processing times, fouling problems, and the discharge of toxic by-products [14].

Adsorption is an alternative technique that stands out from other methods due its simple operational features, high efficiency, lower costs, and diverse nature of adsorbent materials. It involves the build-up of adsorbate through chemical and/or physical binding via a fluid phase onto the adsorbent surface [7]. The adsorption process can be utilized for the removal of organic and synthetic contaminants of low concentrations from water [15]. Also, it is useful in the eradication of compounds with lowto medium molecular masses without the formation of unwanted by-products. Adsorption can be deployed as a standalone process or integrated into other technologies to enhance the treatment performance. Its scalability, effectiveness, and simplicity at trace-level concentrations in complex water matrices make it an attractive choice for the tertiary treatment of surface water [9].

This review will provide an overview of the occurrence and impact of pharmaceuticals in surface waters, as well as the use of agricultural materials for their removal through adsorption. Only a limited number of reports have been documented on the utilization of adsorbents for the removal of pharmaceutical contaminants from surface water [16]. This review aims to summarize the occurrence of pharmaceuticals in surface water, as well as the associated health and environmental impacts. This review identifies sustainable low-cost adsorbents and discusses their characteristics, adsorption efficiency, and merits as materials suitable for water treatment. The novelty of this review is in highlighting the benefits of low-cost adsorbents, which have a high adsorption capacity for the removal of pharmaceutical contaminants, compared to activated carbon has been utilized for the adsorption of a variety of compounds due to its microporous structure and high surface area [17,18,19]. However, activated carbon cannot be considered as a low-cost adsorbent due to its expensive nature. The various methods of modifying these adsorbents to improve their efficiency will be discussed. Also, the novelty of the review lies in the integration of several perspectives on low-cost adsorbents for the removal of pharmaceuticals from surface waters. It involves early utilization of raw agricultural residues and clay, valued for their abundance and the low cost of materials but limited in capacity, selectivity, and regeneration potential. Present-day engineered materials include magnetic biochar and biopolymer clay that could achieve better performance and easier recovery. The bridge between the past and present could be strengthened by assessing how real-water conditions, including inorganic ions and competition from natural organic matter, could interfere with the laboratory performance of adsorbents. This review provides factual context and a plan for developing a potent field-ready adsorbent for sustainable water treatment.

The aim in selecting studies for this literature review was to reflect on the current state of the art on the adsorption of pharmaceuticals from surface water using low-cost adsorbents derived from agricultural by-products and other sustainable precursors. Peer-reviewed studies published within the last decade were prioritized. Studies were retrieved from Scopus, Google Scholar, and Web of Science using key words such as ‘pharmaceutical adsorption’, ‘low-cost adsorbent’, ‘agricultural waste’, and ‘surface water’. Studies on high-cost materials such as activated carbon were excluded.

2. Pharmaceuticals in Water

Pharmaceuticals are compounds composed of various active components used to prevent and treat various diseases in humans and animals [20]. Their occurrence depends on factors that include the degree of industrialization, pattern of consumption, and environmental regulation. Pharmaceuticals differ from other contaminants of chemical origin as their formation involves numerous convoluted molecules that vary in structure, molecular mass, and functionality. Some pharmaceutical compounds are moderately soluble in water and lipophilic in nature: these are polar compounds with their characteristics dependent on the medium’s pH levels. Sulfamethoxazole, naproxen, and erythromycin are examples of drugs that might persist in the environment for over a year, and clofibric acid might persist for several years and accumulate to become active [11]. The existence of these pharmaceuticals in aquatic bodies has dramatic effects on the environment and a host of organisms [21].

Antibiotics. Antibiotics are drugs used to inhibit the actions of microorganisms such as viruses, bacteria, and fungi. Antibiotics are used for the treatment of various infections and diseases in humans and livestock [20]. Over 250 distinct types of registered antibiotic uses for veterinary and human treatment have been reported [20]. The most common antibiotics include macrolides, tetracyclines, sulphonamides, quinolones, and penicillins. There is a regular increase in the production of antibiotics around the world, as their total usage increases from 100,000 to 200,000 tonnes per a year. The global antibiotic consumption rate is predicted to grow by 200% by the year 2030 in both low- and middle-income countries [21]. Despite their health benefits, the discharge of antibiotics into the environment and health risks to humans are of concern due to the increase in antibacterial resistance [21].

Hormones. Hormones are synthetic analogues of plant- and animal-derived hormones that have an effect on endocrine function. Estrogen is the most common hormone found in the aquatic environment. Synthetic estrogen is utilized in estrogen therapies and birth control drugs. Estrogen and its metabolites such as estrone and 17β-ethinyl/estradiol are responsible for adverse developmental and reproductive effects in aquatic organisms [22].

Analgesics and Anti-inflammatories. Analgesics are a collection of drugs used for pain relief and anti-inflammatories arefor inflammation treatment. Examples of drugs belonging to the anti-inflammatory class include diclofenac, naproxen, and ibuprofen. Aspirin and acetaminophen are well-known analgesics. Due to their persistence and stability in ground and surface water, they are classified as emerging contaminants [23].

β-Blockers. These drugs are used for the treatment of cardiovascular diseases such as hypertension, arrhythmia, and angina. β-blocker drugs include propranolol, atenolol, and metoprolol. One of the most consumed β-blockers is metoprolol; its metabolism generates metoprolol acid and several derivatives that account for more than 70% of urine content [24]. Furthermore, other types of pharmaceuticals are listed in Figure 2.

2.1. Occurrence of Pharmaceuticals in Surface Waters

For decades now, more than 3000 pharmaceuticals and metabolites have been known and utilized for the management of several diseases. Pharmaceutical compounds have two possible sources, which can be classified into diffuse sources and point sources. Point sources include septic tanks and municipal, industrial, and hospital effluents that account for most of the pharmaceuticals in several environments, while diffuse sources include urban run-offs, wastewater treatment plant leakages, and agricultural practices run-off also contributing to the load of pharmaceuticals in the environment [25,26]. For example, in India, pharmaceuticals that include citalopram, ciprofloxacin, enoxacin, ofloxacin, and metoprolol have been detected in sewage treatment plant effluents [27]. In South Africa, ibuprofen, naproxen, and diclofenac have been identified in aquatic bodies from the Mbokodweni River [28]. Phenazone, piroxicam, indomethacin, and ketoprofen have been identified in Greek wastewater treatment plant effluents and influents [29]. Phenyton and sulfamethoxazole were identified in raw water samples sourced from several drinking wells in Massachusetts, USA [30]. In Ireland, diclofenac, nimesulide, metoprolol, clotrimazole, and carbamazepine have been identified at several wastewater plants [31].

Sufamethoxazole, acetaminophen, carbamazepine, and amoxicillin, among other pharmaceuticals, were also identified in surface water and effluents [32,33]. Metaformin, fexofenadine, gabapentin, and tramadol were detected in two rivers (Foss and Ouse) in the United Kingdom [34]. In the Kajangand area of Malaysia, the presence of pharmaceuticals such as sulphamethoxazole, chloramphenicol, ciprofloxacin, and amoxicillin has been detected in drinking water [35,36]. The continuous presence of these pharmaceuticals in various aquatic bodies in trace concentrations could be toxic to ecosystems and organisms over a long period of time [37,38].

Table 1. The occurrence of pharmaceuticals in surface waters around the world.

Compounds	pKa	Minimum Level (ng/L)	Maximum Level (ng/L)	Country	Reference
Antibiotics
Metronidazole	2.38	0	13.51	Bangladesh	[1]
		0	5.10	China	[2]
Trimethoprim	7.12	0	17.20	Bangladesh	[1]
		0	15.70	China	[2]
		0	290	South Africa	[3]
		0	76	UK	[4]
		0.40	52.10	China	[5]
		0	2.29	USA	[6]
		34	74	Mexico	[7]
		0.02	0.33	Sweden	[8]
Erythromycin	8.88	10.20	183	China	[5]
		0	240	South Africa	[3]
		0	6.46	Bangladesh	[1]
		0	263	UK	[4]
		32.89	38.80	Portugal	[9]
Sulfamethoxazole	1.6; 5.7	2.83	20.80	China	[5]
		108	502	Mexico	[7]
		0	0.14	Sweden	[8]
		0	2.50	China	[2]
		19.26	114.24	Malaysia	[10]
		0	5320	South Africa	[3]
		0	14.73	USA	[6]
		0	7.24	Bangladesh	[1]
		0	43	Portugal	[39]
Antiepileptic medications
Carbamazepine	13.9	0	3.50	China	[5]
		0	36	Mexico	[7]
		0	5.80	France	[11]
		0.94	9.39	USA	[6]
		0	8.80	Bangladesh	[1]
Central nervous system (CNS) stimulants
Caffeine	14	0	2640	India	[11]
		0	220	China	[2]
		0	81	France	[11]
		8.05	26.92	USA	[6]
		0	332	South Africa	[3]

Note: pK_a—acid–base ionization constant.

presents a list of various pharmaceuticals that have been detected in several surface waters across the world.

2.2. The Impact of Pharmaceuticals on the Environment and Human Health

Various forms of pharmaceuticals and their metabolites are released into aquatic bodies; also, the consumption of water in the form of drinking water from these aquatic habitats in some regions of the world and the inefficiency of WWTPs in removing pharmaceuticals effectively have an adverse effect on ecosystems in aquatic bodies and human health [40,41]. The detrimental impact of these pharmaceuticals on aquatic bodies’ ecosystems cannot over emphasized. The issue of antibiotic resistance is alarming and increasing due to the inefficiency of WWTPs. Continuous and long exposure of aquatic habitats to pharmaceuticals results in the impairment of physiological functions and ecotoxicity [42,43].

The long-term exposure of humans to these pharmaceuticals results in serious health challenges, which include mutation, teratogenesis, and cancer [44,45,46]. It is important to evaluate the various impacts of pharmaceutical compounds and their exposure and investigate sustainable treatment techniques for the efficient removal of pharmaceuticals.

3. Adsorption

Adsorption is a proven technique used for treating several kinds of effluents in the environment. Adsorption involves the the addition of an adsorbate at the intersection of phases, which include gas–solid, liquid–liquid, liquid-solid, and gas–liquid interfaces, onto the adsorbent surface via chemical or physical binding [47,48]. The adsorbate is the substance being adsorbed, while the adsorbent is the material adsorbing the adsorbate. Figure 3 explains the terminology relating to the adsorption process. The properties of the adsorbent and adsorbate are specific and dependent on their constituents. Adsorption might occur as a chemisorption or a physiosorption process. The inequality of attraction forces on the adsorbent surface propels the adsorption process. Physiosorption involves the synergy between physical adsorbed molecules and solid surfaces. The Coulomb and Van der Waals interactions ensure physiosorption by inducing synergy between adsorbate and adsorbent molecules. Also, physiosorption is an exothermic and reversible process [49,50]. Chemisorption involves the attraction of adsorbent molecules and absorbate driven by chemical bonding. Figure 4 presents the possible adsorption process pathways. Chemisorption is an endothermic and irreversible process [51]. Both chemisorption and physiosorption could proceed alternatively or simultaneously under appropriate conditions.

Adsorption is an undeviating and safe approach that encourages sustainable environmental research [53]. Adsorption has been applied in the sequestration and separation of materials at refineries during environmental, mining, and industrial operations. Several treatment techniques for the removal of pharmaceuticals have been studied and documented, but as a technique, adsorption has attracted appreciable interest in dealing with pharmaceutical contaminants. Adsorption is a cost-effective, efficient, and versatile technique for the elimination of a variety of water pollutants [54]. Activated carbon [55], zeolites [56], biochar [57], carbon nanotubes [58], mesoporous silica [59], chitosan [60], anion exchange resin [61], graphite oxide [62], clays [63], and biomass waste [64] have been utilized effectively for the elimination of pharmaceuticals.

3.1. Classification of Adsorbents

Adsorbents are materials with the capability to eliminate toxic pollutants from gases or liquids. Adsorbent efficiency is dependent on the adsorbent’s surface physico-chemical features and adsorbate solubility [65]. The other properties of an adsorbent include mechanical stability, high porosity, large surface area, chemical and thermal strength, low diameter, and a small volume of pores. Also, enhanced surface chemistry will result in an effective adsorption capacity [66]. Adsorbents can be classified into industrial wastes, biosorbents, natural materials, agricultural wastes, and modified organic resources. Figure 4 shows some examples of low-cost adsorbents that have been used for the removal of pharmaceuticals. However, this review discusses the utilization of agricultural materials, biochar, clay, rice husk, and other low-cost materials for the removal of pharmaceuticals. Additional classifications have been proposed, which include conventional and non-conventional adsorbents [67]. Conventional adsorption materials could be composed of activated carbon, silica, zeolites, and activated alumina. Non-conventional adsorption materials include activated carbon from agricultural waste, organic resources, and manufacturing by-products [67].

3.1.1. Low-Cost Adsorbents

Low-cost adsorbents include materials derived from inexpensive, abundant, and available raw sources such as agricultural residues and some industrial by-products [67]. Also, low-cost materials do not necessarily translate to low management costs at full scale. Operational factors such as the adsorbent lifetime, energy, reagent, and regeneration efficiency can influence total treatment costs [67]. Lignocellulosic materials derived from biomass have been studied as bio-adsorbents for the elimination of water contaminants [68]. Above 165 million metric tonnes of lignocellulosic materials per year are generated throughout the world [68]. Lignocellulosic materials are made up of three major polymeric components: lignin, hemicellulose, and cellulose. They are found in varied proportions that are dependent on the biomass origin [69]. The composition and the source of lignocellulosic materials vary due to their geographical area, as well. The compositional make-up of lignocellulosic materials determines their adsorbent functionality, which allows for the development of functional adsorbents for the elimination of water contaminants. Lignocellulosic materials possess some fascinating attributes such as high adsorption capacity, low costs, biodegradability, reduced toxicity, and eco-friendliness, which make them a preferred alternative to expensive activated carbon adsorbents.

Lignin

Lignin is a renewable polymer that is the most bountiful in nature after cellulose. Lignin’s structure comprises a three-dimensional, non-crystalline, and complex phenolic network found in plants. Lignin is made by polymerization of three precursors including p-coumaryl alcohol, sinapyl alcohol, and coniferyl alcohol, which provide hydroxyphenyl (H unit), syringyl (S unit), and guaiacyl (G unit) to the structural units of lignin. The content of these units differs across various classes of plants. Lignin has various functional groups and structures such as carbonyl -C=O, carboxyl -COOH, hydroxyl -OH, aromatic rings, and methoxyl–OCH₃, which may act as binding sites during adsorption [70]. The hydroxyl groups of lignin could exist in the form of ether bonds attached to aryl fragments or alkyl groups. Lignin can form intra- and inter-molecular hydrogen bonds due to the presence of hydroxyl groups. Commonly, the methoxyl groups are bonded to the benzene rings. Also, the variety of functional groups and aromatic structures give lignin its reactive attributes [71]. The presence of phenolic structures and a system of hydrogen bonds impact lignocellulose’s resistance to chemical or enzymatic hydrolysis. Also, lignin is biodegradable non-toxic and has antimicrobial and antioxidant properties [72]. Lignin can function as an adsorbent in both modified and native forms. Also, lignin’s amorphous chains are able to create inter- and intra-molecular linkages that might result in lower adsorption capability [73].

Hemicellulose

Hemicellulose is an abundant heterogeneous polysaccharide; it accounts for 20–35% of lignocellulose’s total composition [74]. Hemicellulose has a branched structure; the backbone can be either a heteropolymer or homopolymer of short branches joined via β-(1,4)-glycosidic bonds and β-(1,3)-glycosidic bonds. Hemicellulose may contain branches of shorter lateral chains of various monosaccharides, which include pentoses, hexoses, and uronic acids. Also, these components differ depending on the lignocellulose source [75]. Hemicellulose polymeric chains contain various reactive functional groups that include carboxyl, hydroxyl, methoxyl, and carbonyl groups that can be modified via chemical reactions, appropriating specific properties to hemicellulose macromolecules [68]. Hemicellulose derivatives such as acetylated, carboxymethyl, oleoylated, cationic, and lauroylated hemicellulose have been synthesized to incorporate several functional groups [76]. These derivatives could be used in the preparation of functional adsorbents with efficient adsorption capacity for the elimination of water contaminants.

Cellulose

Cellulose is a bountiful polysaccharide found in nature and serves as an alternative for the elaboration of several materials due to its chemical stability, biodegradable ability, and non-toxic attributes. Cellulose is a linear polymer of β-D-glucopyranose units linked via 1,4-β-glycosidic bonds. Intramolecular interactions exist among the cellulose units adjacent to each other. Also, each unit contains three hydroxyl groups and six carbon atoms [77]. The structural make-up of cellulose can be split into a cellulose chain, a supramolecular cellulose layer massed via the crystalline form of cellulose, and a fibril layer. The fibril layer is contrived by the amorphous and crystalline region and various pore structures assembled during the process of cell self-assembling [68]. Cellulose macromolecules contain hydroxyl groups, which are involved in hydrogen bonding, forming cross-linked networks. Cellulose hydroxyl groups could be modified to incorporate distinct functional groups such as carboxyl and sulfonic groups to provide efficient removal of water contaminants [78]. The modified cellulose adsorbents have an improved adsorption capacity in comparison to unmodified cellulose [78]. There are several chemically modified cellulose derivatives, which include hydroxypropyl cellulose (HPC), methylcellulose (MC), carboxymethylcellulose (CMC), and quarternized hydroxyethyl cellulose ethoxylate (HEC), that have been utilized as adsorbents [79]. Cellulose has been utilized as an efficient adsorbent at WWTPs in various forms such as composite adsorbents, nanoparticles, nanocomposites, hybrid cellulose, and hydrogels [80].

Agricultural Wastes

Agricultural waste materials are the by-products of diverse crop practices and cultivation. Components of agricultural waste materials include lignin, cellulose, proteins, hemicellulose, lipids, water, proteins, starch, simple sugars, and hydrocarbons. Agricultural waste materials can be utilized either in modified or natural forms. Materials in their natural form are washed, followed by blending and sieving until the desired particle size is achieved, while modified forms involve the pre-treatment of materials via modification techniques [81]. Pre-treatments help to reinforce and enhance the potential of functional groups and improve the number of active sites. Agricultural waste materials have been widely considered for use as low-cost adsorbents due to their cost effectiveness, abundance, and capability of eliminating organic and inorganic contaminants from wastewater.organic and inorganic contaminants from wastewater.

Rice husk is an agricultural waste of high abundance. It is utilized as an adsorbent due to its insolubility in water, mechanical strength, granular structure, and availability. Rice husk can be treated with acids to break down the lignin, cellulose, and hemicellulose components to improve its surface area and porosity and enhance its adsorption efficiency [82]. Due to the presence of silanol groups of silicic acid on its surface, which improve its cation exchange capability, rice husk is effective for the removal of cadmium, iron, arsenic, and manganese from an aqueous medium. It has been observed that rice husk is effective in the elimination of antibiotics from aqueous media [82]. For example, 4.8 mg/g of metronidazole and 7.7 mg/g of levofloxacin were eliminated from water via activated rice husk [83].

Sugar cane is widely cultivated throughout the tropical regions of the world. Sugar cane bagasse is a product of sugar cane that can be utilized for power generation, paper manufacturing, and fuel for boilers. Sugarcane bagasse is composed of cellulose (2%), lignin (20%), and hemicellulose (25%) [84]. The authors of [84] investigated the elimination of Mn (III) and colour from water via sugar waste material and raw sugar waste. Furthermore, it has been reported that sugarcane bagasse has a high removal capacity for Ni (II) (123.46 mg/g) compared to Pb (II) (1.61 mg/g) [85]. Thermally and chemically activated sugarcane bagasse had improved pores and surface area above 1500 m²/g with a maximum adsorption capacity of Pb (II) 19.3 mg/g, Ni (II) 2.99 mg/g, and Cu (II) 13.26 mg/g [86].

Fruits are produced in huge quantities around the world. It is estimated that over 599.3 million tonnes of fruits and fruit peels are generated. The adsorption of contaminants on fruit peels has been evaluated due to their efficiency, cost-effectiveness, sustainability, and availability [87]. Fruit peels contain carbon-rich compounds that include hemicellulose, pectin, cellulose, chlorophyll pigments, and compounds of low molecular weight that enhance the adsorption of pharmaceuticals, heavy metals, and emerging contaminants [87]. Watermelon rind was investigated for the removal of arsenic from ground water, and the removal efficiency was 99% [88]. Peels of pineapple, pitaya, and orange were synthesized into activated carbon for the elimination of ammonium from water. The adsorbents were synthesized at temperatures of 573, 673, and 873 K within a time frame of 2 h and 4 h. Pineapple and orange peel showed high adsorption efficiency in comparison to pitaya peel [89].

Sawdust is a biowaste of agricultural origin with diverse applications. Sawdust biowaste can be utilized in the management and remediation of pollutants. Sawdust is composed of lignin (23–30%), cellulose (45–50%), hemicellulose (20–30%), and several extractives such as soluble sugar, resins, acids, oils, and waxes (5–15%) [90]. Sawdust contains several functional groups and structures such as hydroxyl, carboxyl, and phenols. Sawdust offers high porosity, mechanical stability, low specific gravity, biodegradation, good liquid retention capacity, easy modification, and good carbon content, making it a valuable adsorbent [91]. A comparison between unmodified sawdust and modified sawdust showed that the base-treated sawdust had the maximum adsorption toward tetracycline [92]. A modified mixture of birch sawdust and Eucalyptus (1:4) using sodium hydroxide, water, and triethanolamine (40:150:30) was assessed for the removal of contaminants in lubricant waste. The modified sawdust had a higher surface area compared to the raw sawdust [93].

Sulfonated sawdust, which involves the modification of sawdust material with sulfuric acid, has been utilized for the removal of pharmaceuticals such as tetracycline, methoxazole, and vancomycin [94]. The sulfonated sawdust adsorption capacity for methoxazole was 295.06 mg/g, for tetracycline was 270.53 mg/g, and for bisphenol A was 263.75 mg/g. The sulfonated sawdust maximum adsorption capacity was highly comparable to that of commercially modified carbon [94]. The chemically activated Onopordum acanthium biomass adsorption capacity for diclofenac and naprofen was evaluated by [94]. Nitrogen adsorption–desorption isotherms revealed that treatment of biomass with nitric acid and potassium hydroxide reduced the pore size and specific surface area of samples. Thermo-gravimetric analysis (TGA), isoelectric point (pH_zphc), and Boehm’s titration analysis revealed the presence of lactone and phenolic groups on the modified biomass surface, indicating that an increase in acidic strength is dependent on the carbon content.

Biochar

Biochar is a constituent of carbonaceous and heterogeneous material derived from biomass via a thermochemical process called pyrolysis, which involves the decomposition of organic compounds at high temperatures and in the absence of oxygen to produce a vapour and solid residual component called biochar. Biochar is a promising material for the elimination of heavy metals and inorganic and organic contaminants [95]. The types of pyrolysis can vary from intermediate to flash pyrolysis and from fast to slow pyrolysis [96]. Slow pyrolysis has been adopted for most studies due to producing the highest yield of biochar. Biochar properties such as elemental composition, surface area, pore structure, and surface functional groups are dependent on precursor and carbonization conditions. Carbonization conditions such as heat transfer, pressure, temperature, and reaction time can influence biochar characteristics [97]. A higher temperature of pyrolysis increases the surface area due to organic matter degradation, the removal of pore-breaking substances, and thermal cracking [98]. Biomass becomes deoxygenated and dehydrated at high temperatures, resulting in lower levels of oxygenated and hydrogen-containing functional groups [99]. Biomass pyrolyzed at temperature above 600 °C generates biochar with high stability, microbial decomposition resistance, and elevated aromatic carbon content. Biochar derived from animal manure has increased ash content and a high pH value as a result of its feedstock nutrient value [100]. Carbonization at a medium temperature (400–700 °C) generates biochar with high aromaticity, high porous development, and increased electron donor attraction [101]. Moderate carbonization at medium temperatures produces biochar that might contain nitrogen and oxygen atoms in the surface groups, which improve functionalities and act as electron acceptors. Biochar generated at temperatures above 700 °C is recalcitrant and hydrophobic and might possess no functionality due to the loss of hydrogen and oxygen in its structure [99].

Biochar adsorption capacity can be improved via several modification techniques (physical or chemical involving hematite, magnetite, hydrous manganese dioxide, zero-valent iron, and manganese oxide), which can be applied before and after pyrolysis [102]. Steam treatment, which is a physical technique, improves biochar porosity and introduces oxygenated functional groups, while basic functional groups can be introduced via heat treatment. Chemical modifications involve the utilization of acids or alkali to improve the basic and acidic nature of the biochar.

Table 2. Agricultural wastes used for biochar production and the adsorption of antibiotics from water.

Biochar Feedstock	Antibiotic	Modification Agent	Specific Surface Area (m2/g)	pH	Adsorption Capacity (Mg/g)	Reference
Poplar	Tetracycline	KOH	1.61	5	21.17	[106]
Pine wood	Daptomycin	Fe3O4	634.218	4	212.77	[107]
Reed stalk	Florfenicol	FeSO4⋅7H2O	254.6	6	9.29	[108]
Banana	Furazolidone	FeCl2⋅4H2O	116.97	7.5	37.45	[109]
Corn stalk	Enrofloxacin	KOH	22.69	3	58.29	[110]
Bamboo	Sulfamethoxazole	Fe2O3	61.48	6	212.8	[111]

shows various types of biochar derived from agricultural wastes, using modification agents, for the adsorption of antibiotics. Also,

Table 3. Comparison of specific surface area between original biochar and modified biochar.

Biochar Feedstocks	Reagents of Modification	Specific Surface Area (m2/g)		Reference
		Before	After
		Modification	Modification
Bamboo	Fe2O3	24.56	61.48	[111]
Potato stems and leaves	KMnO4	99.43	252.00	[112]
Corn stalk	H3PO4	3.01	14.17	[113]
Willow wood	Fe3O4	607.076	662.066	[107]
Reed stalk powder	FeSO4⋅7H2O	58.75	254.6	[108]

shows the effect of modifying agents on the surface area of biochar. The impregnation technique utilizes magnetite, hydrous manganese dioxide, hematite, calcium oxide, and zero-valent iron [102]. Studies showed that the biochar of potato stem and leaf had an adsorption capacity of 6.94 mg/g for norfloxacin, 8.37 mg/g for ciprofloxacin, and 7.19 mg/g for enrofloxacin [103]. Biochar from rice husk, compost, orange waste, and olive pomace pyrolyzed at 600 °C had an adsorption capacity of 0.3, 3.4, 0.4, and 0.7 mg/g for Cu (II), respectively [104]. Modified biochar has shown the potential for the elimination of pharmaceuticals. Rice husk biochar and empty fruit bunch had adsorption capacities of 19.3 and 18.9 mg/g for tetracycline at pH 9. The iron coating of rice husk biochar and empty fruit bunch had an increased adsorption capacity of 30.8 and 31.4 mg/g for sulfamethoxazole. Biochar derived from Opuntia ficus indica cactus fibres and modified with nitric acid had an adsorption capacity of 3.5 mol/kg for Cu (II) at a pH of 6.5 [105].

Clay

Clays are natural, small particles of hydro-aluminum silicate deposits found in abundance on Earth. They are composed of silica, alumina, weathered rocks, and water. Also, they are composed of cations of alkaline earth and alkaline metals [107]. The physical properties of clay include good plasticity, fine particles, high refractoriness, hardness, and surface decoration capacity. Clays possess a high surface area, smaller particles sizes, and highly porous architecture, which influence their chemical and physical interactions with various species. These interactions occur due to crystallinity, adsorption cation exchange, and electrostatic repulsion. Clays are composed of three different edges: the outer and inner surfaces amid layers of silicate. The outer surface interlayer tends to undergo modification during adsorption and ion exchange processes. Isomorphic substitution influences clay minerals to generate a small quantity of net negative charge. Also, the layers of clay mineral particles might give off charges dependent on the solvent’s pH due to the dissociation of the primary bonds, which include Al-O and SI-O [107]. Clay minerals possess features such as octahedrally and tetrahedrally coordinate atoms, which form octahedral and tetrahedral silicate sheets that are below 2 mm in thickness. Tetrahedral sheets are composed of SiO₄ tetrahedral layers involving the participation of three basal oxygen atoms, giving rise to a hexagonal pattern. The octahedral sheet contains Mg, Fe, or Al cations concerted via six equal distant hydroxyl groups or oxides [108].

According to the structure of the internal layer of clays, they can be classified into two forms: crystalline and amorphous. Crystalline clay is composed of crystal structures of various groups, which include a 1:1 type tube (halloysite), a 1:1 type layer (kaolinite), a regular mixed-layer type (chlorite group), a 2:1 type layer chain (attapulgite, sepiolite), and a 2:1 type layer (smectite, montmorillonite, and vermiculite) [107]. Kaolinite with chemical composition Al₂Si₂O₅(OH)₄ contains SiO₂ (46.53%), H₂O (13.94%), and Al₂O₃ (39.53%). Kaolinite possesses a 1:1 layer structure composed of octahedral and tetrahedral sheets. Despite kaolinite’s overall neutral surface charge, the broken edges possess a relatively negative net charge. Kaolinite has been used in the cosmetics, medicine, paper, and ceramic industries and also in water treatment applications. Kaolinite has been applied to various clay minerals and induces the release of H⁺ ions from the edge of the structure layer in an acidic medium, promoting adsorption [107]. Halloysite (Al₂Si₂O₅(OH)₄ × 2H₂O) has a 1:1 layer with hollow and nanotubular microstructures. The phyllosilicate and crystalline architecture of the nanotubes are fashioned via two components, which include the sharing of a Si-O tetrahedral sheet around the corner and the sharing of an Al-O octahedral sheet on the edge. Furthermore, just two thirds of the octahedral sites in the Al-O octahedral sheet are loaded with aluminum [107]. The interlayer cation, cation exchange capacity, and surface area influence the biosorption and adsorption processes involved in the elimination of pollutants from water [114]. The inner and outer lamellar layers of clay can be modified to produce an adsorbent that can be applied for the removal of diverse contaminants from water [115]. Modification of clays allows for the addition of new functional groups and surface hydrophobicity improvement, which influences inorganic cation exchange between interlayer spaces of clay and diverse materials such as pharmaceuticals [116]. The interaction of cellulose with modified sodium surfactant, which influenced the adsorption of Cr (VI) as a bichromate anion from the aqueous solution onto the clay bi-composite surface, was reported in [117]. Swelling, which is an important mechanical behaviour, can be influenced by the osmotic pressure, affinity, and cross-linking density of the clay. Furthermore, molecular dynamics was utilized for a study on the effect of the layer charge location, temperature, and interlayer cation on intra-crystalline clay mineral swelling. The study [118] shows that the temperature increase induced a swelling curve shift to a higher d-spacing value on the interlayer structure with slight effect [118]. The literature has reported the utilization of magnetic halloysite nanotubes for the removal of Pb (II), which achieved a removal rate of 23.1 mg/g [119,120].

Clay activation can be carried out to improve clay character for desired properties. Physical and chemical treatments, which include acid, alkaline, and thermal activation, have been studied. Thermal treatment is expensive and disruption of the interlayered structure of clay often occurs. Alkaline activation improves the water-resistant capability and physical and mechanical properties of clay. Also, it diminishes its capillarity [121]. Acid activation is a simple process that enhances the characteristic attributes of clay for efficient performance. Clay pore volume and surface area could be improved via mineral acid treatment. The activation of bentonite with hydrochloric acid showed enhanced surface porosity. Brunner–Emmett–Teller (BET) analysis shows that the modified bentonite had 65.89 m²/g surface area, and that of pristine bentonite was 50.93 m²/g [122]. Also, for the use of sulfuric acid for the activation of Moroccan clay for 12 h at 110 °C, the BET results revealed that the activated bentonite had a surface area of 74.43 m²/g, while the pristine clay had a surface area of 51.42 m²/g [123]. The challenges involved in the utilization of clay adsorbent for water treatment processes include low recovery, clogging, low flow rates, and adsorbent decomposition when left in the solution.

4. Physico-Chemical Factors Affecting Adsorption

4.1. Effect of Temperature

The adsorption process is significantly influenced by temperature. It predetermines whether an adsorption process is exothermic or endothermic. Temperatures within the range of 20–35 °C could affect the adsorption process moderately [124]. Lower temperatures favour the exothermic adsorption process, while higher temperatures favour the endothermic process. Endothermic processes could occur due to an increase in adsorbate molecule mobility due to temperature increase, which influences the increase in adsorbate–adsorbent attraction [125]. For the adsorption of pharmaceuticals, activation of adsorption active sites could be due to an increase in temperature [126]. Reports on endothermicity include the adsorption of p-nitrophenol by pine sawdust [127]; regarding the adsorption of diclofenac by pine wood biochar, it was recorded to have a 99.6% removal efficiency at 500 μg/L under an endothermic condition [128]. In the adsorption of 2,4,6-trichlorophenol on modified kaolinite, there was an increase in adsorption as the temperature increased from 20 to 50 °C [126]. The temperature increase might reduce the adsorption capacity in the case of exothermic processes; this might be due to the reduced attraction forces between the adsorbate and adsorbent [129]. There is a proportional relationship between molecular movement acceleration and temperature, which enhances adsorbent–ion attraction and decreases free Gibbs energy. Reference [130] explained that free Gibbs energy was observed to decrease as the temperature increased from 30 to 50 °C. This resulted in spontaneity in the process due to the temperature increase favouring the adsorption process. Furthermore, increasing the temperature promoted aromatic carbon formation on the adsorbate surface, resulting in enhanced surface characteristics [130]. aromatic carbon formation on the adsorbate surface, resulting in enhanced surface characteristics [131].

4.2. Effect of pH

The pH level is important due to its influence on the adsorption process, especially on electrostatic interactions between the surface of adsorbents and molecules of adsorbates. Furthermore, the pH impacts the charge characteristics, via protonation–deprotonation of oxygen-containing groups, of the adsorbates [131]. The pH influences the adsorption of pharmaceuticals via its effect on the functional groups, ionization state, and surface charge of the adsorbent. Furthermore, pH value affects the characteristics and ionic forms of pharmaceuticals [132]. Pharmaceuticals adsorption in the binary system is similar to that in the unitary system and suggests that pharmaceuticals are better adsorbed under weak acidic or neutral conditions in both the unitary and binary systems. When the pH is lower than the pH_pzc (point of zero charge) of the adsorbent, the positive charge on the adsorbent surface increases, and vice versa. For example, it has been reported that the removal of diclofenac is pH-dependent, with the most efficient removal achieved at pH 2 [25]. Also, olive residue biosorbent at high pH levels had a reduced uptake of ibuprofen and diclofenac [133].

4.3. Effect of the Adsorbate Initial Concentration

The adsorbate initial concentration influences the efficiency of the adsorption process. An increase in the initial concentration of adsorbate decreases adsorption capacity. This is due to the ability of the adsorbate to occupy the available active site on the surface adsorbent material [129]. Saturation of adsorption sites on the adsorbent surface results in a decrease in removal percentage with an increase in initial concentration. Unsaturation of the adsorption sites on the adsorbent surface will increase the removal percentage with an increase in initial concentration because an increase in initial concentration provides a high mass transfer driving force for adsorption, which improves the diffusion rate and increases the probability of molecules reaching the adsorbent surface [100]. It has been reported that decreased adsorption efficiency is proportional to increasing the ciprofloxacin initial concentration. This is due to the progressive saturation of the active site for antibiotic molecule adsorption [134].

4.4. Effect of Adsorbent Dosage

An increased amount of adsorbent results in a higher removal efficiency due to the increase in the number of adsorption sites. Adsorbent dosage influences the availability and count of binding sites available for the adsorbate. An increased dose of adsorbent could result in higher contaminant removal. Consequently, an excess of adsorbent could result in its agglomeration, thus resulting in surface area reduction, and, hence, in reduced removal efficiency [135].

4.5. Effect of Contact Time

The contact time between adsorbate molecules and the adsorbent influences the rate at which adsorption equilibrium can be achieved. Adsorption capacity increases rapidly at the beginning of water treatment, then, it slows down and regresses steadily at the equilibrium stage [100]. This could be explained by the step-wise saturation of active sites of the adsorbent and the decrease in concentration between the adsorbent surface and bulk phase of the solution/water [136]. For example, it was reported that the adsorption of ciprofloxacin by adsorbent modified with phosphoric acid (H₃PO₄) reduced the adsorption rate of ciprofloxacin molecules with increasing treatment time until the equilibrium point was achieved [134].

5. Adsorption Mechanisms

5.1. Hydrogen Bonding

The hydrogen bond pattern between the surface groups of adsorbent and adsorbate is a means by which pollutants can be eliminated from solutions. Hydrogen bonding involves a physical bond between the hydrogen atom and the oxygen atom, separated by a short distance, less than 3.5 Å, with an angle less than 30° [137]. Hydrogen bond formation occurs between functional groups of adsorbent and adsorbate, which may include carboxyl (-COOH), carbonyl (-C=O), amino (-NH₂), and hydroxyl(-OH) groups [138]. Hydrogen bond formation could be ascertained via density formation theory (DFT) and Attenuated Total Reflectance–Fourier Transform Infrared (ATR-FTIR) spectroscopy [139]. For instance, the FTIR technique was utilized to study the chlortetracycline adsorption process on nano-iron encased with graphitic carbon through a hydrogen bond mechanism [140]. The C-H and O-H bands in the spectrum of graphitic carbon after chlortetracycline adsorption on its surface were shifted as a result of the interaction by hydrogen bonding between graphitic carbon and chlortetracycline. Also, the adsorption of ofloxacin on nanoparticles of magnetic iron was described as occuring through hydrogen bond interactions [141]. The authors, after observing the FT-IR signals before and after the antibiotic adsorption on the magnetic irons, stated that the C-O and O-H signals shifted from 1398 cm⁻¹ to 1383 cm⁻¹ and 3424 cm⁻¹ to 3422 cm⁻¹, respectively. This was due to hydrogen bonding mechanism between ofloxacin and the adsorbent.

Some publications have reported the utilization of modified and raw biochar obtained from rice husks for the elimination of tetracycline. Adsorption of tetracycline via surface modification was evaluated, and the mechanism of tetracycline elimination was proposed to occur via the attraction of modified biomass surfaces and tetracycline acidic ions resulting in hydrogen bond formation [142]. Coffee husk and rice husk have been utilized as organic adsorbents for the elimination of norfloxacin from aqueous solutions. High porosity and small surface area have been observed to be among the adsorbent characteristics. The mechanism responsible for the adsorption of norfloxacin onto organic adsorbents was thought to happen via hydrogen bonding, as well [143].

5.2. π-π Electron–Donor–Acceptor Interactions

The π-π interaction happens between electron-rich systems of π bonds and anions, metal cations, or other systems. Strong electrostatic interactions between the negative charge of π electrons and positively charged ions could result in the cation–π interactions, while the interaction of aromatic rings with electron deficiency and negatively charged species could result in the anion–π interactions [144]. The adsorption mechanism involving π-π interactions between an adsorbate and adsorbent is associated with the interaction between a single pair of electrons with a π delocalized system of an aromatic ring [145]. It has been reported that tetracycline adsorption on the maghemite surface occurred via the mechanism of π-π interactions. This was confirmed by the FT-IR spectroscopy of maghemite and tetracycline samples before and after the adsorption process: the tetracycline -C=O band shifted from 1611 cm⁻¹ to 1622 cm⁻¹ after the adsorption of tetracycline on maghemite. This observation suggested an interaction between the tetracycline π electron ring and the maghemite oxygen atom [139]. Another literature source reports on the adsorption of β-estradiol by nano-akaganeite. The adsorption of β-estradiol on the akaganeite molecule could be a result of cation–π interaction amid the π-system of β-estradiol and the positive charge surface of akaganeite [146].

5.3. Hydrophobic Interactions

The hydrophobic interaction mechanism happens between the adsorbate and adsorbent as a result of non-polar molecules’ attraction to each other in aqueous media. Hydrophobic interaction is utilized, for example, in chromatographic techniques when non-polar groups of protein molecules in aqueous media are adhered to a column, thus providing a separation of proteins from impurities [147]. Also, the elimination of contaminants via hydrophobic interaction has been reported in [148]. The hydrophobic interactions promote molecule associations that might be driven by aromatic surface dissolution, which is in synergy and leads to an increase in the translational entropy of water molecule release [149]. Non-polar molecule aggregations occur via an increase in environmental entropy. Also, the aggregation of molecules results in a reduced non-polar surface area (hydrophobic sites), leading to a lower level of order (higher entropy) and to reduced Gibbs free energy [149]. An increased salt concentration in aqueous media of both the adsorbent and adsorbate tends to promote hydrophobic interactions. Conversely, hydrophobic interactions can be reduced by the addition of a salt [150].

The adsorption of ciprofloxacin by Fe₃O₄/C was explained by a hydrophobic interaction mechanism in the work [151]. It was reported that the maximum ciprofloxacin adsorption was measured at neutral pH levels. Also, at pH 8.7 and 6.0, the solubility of ciprofloxacin was reduced, indicating increased hydrophobicity of this antibiotic. In another work, the maximum adsorption of pefloxacin mesylate dehydrate by modified biochar of manganese ferrite was observed at pH 5.0 [152]. The adsorbate may have an increased number of hydrophobic sites for antibiotic adsorption with an increased water partitioning coefficient. Therefore, hydrophobicity could be linked to the solubility of a solute and a higher water partition coefficient [153].

5.4. Electrostatic Interactions

Electrostatic interactions exist between the adsorbate in aqueous media and the charge surfaces of the adsorbent. Also, they determine the coagulation and aggregation rates of particles [154]. Electrostatic interaction is dependent on the adsorbate ionization and surface charge of the adsorbent. The occurrence of electrostatic attraction between contaminant ions and the adsorbent surface with elevated ionic strength might result in a decrease in adsorption capacity. For instance, the electrostatic attraction of acetaminophen to nanoparticles of magnetite modified with β-cyclodextrin has been evaluated [155]. Acetaminophen in an acidic solution is charged positively due to amine group protonation [156]. Between pH 4 and 8, the magnetic polymer surface charge was negative due to the occurrence of electrostatic attraction between the magnetic polymer and acetaminophen. Also, the adsorption of sulfamethoxazole (SMZ) by magnetic biochar was reported to be driven by electrostatic attraction forces [157]. Due to the occurrence of electrostatic attraction between magnetic biochar and SMZ at acidic pH, the SMZ was adsorbed better compared to an alkaline pH level, while the increased ionic strength resulted in increased adsorption due to the effect of salting out. A paper [158] has reported the effect of electrostatic attraction during the adsorption of cephalexin on hematite (α-Fe₂O₃). The adsorption capacity increased with pH growing up to 7.5. This could be due to the hematite negative charge that repelled the electrostatic deprotonation of the negatively charged groups of cephalexin molecules.

6. Surface Modification of Adsorbents

Modification of the adsorbent is aimed at improving its surface by modifying the physical, chemical, or biological characteristics [159]. The modification processes can vary and may involve the utilization of inexpensive reagents. The surface layer of adsorbent materials can be modified via functional coatings and the application of thermal, mechanical, and chemical treatments. The chemical and physical modification of the adsorbent can result in a decreased surface area, as reported in [160]. But, generally, modification increased surface area [160]. Solid adsorbents are modified to vary several surface properties such as the surface charge, surface area, nature of functional groups, surface energy, and hydrophobicity [161]. Mechanical methods could be achieved by milling raw materials to develop pores, with a chemical method to aid surface area improvement [159]. The chemical method involves the application of acids, salts, or alkali to improve surface areas and the access of the adsorbate to functional groups. The physical approach improves physical properties such as density and solubility [159]. A combination of thermochemical and mechanochemical processes could aid in the adsorbent’s surface activation.

6.1. Chemical Modification

This form of modification influences the surface chemistry of the adsorbent. This procedure aids in the transformation of the adsorbent into more valuable materials with higher adsorption efficiency compared to the original material [162]. Chemical modification involves a single-stage procedure in which adsorbent materials are macerated with either acidic or basic agents. For example, Figure 5 illustrates the effect of chemical treatment on sawdust structure.

Acid modification involves the utilization of mineral acids such as perchloric acid (HClO₄), nitric acid (HNO₃), phosphoric acid (H₃PO₄), hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and others. Organic acids such as oxalic, formic, and acetic acid are rarely utilized due to their weak effects and lower strength compared to mineral acids [163]. Acid modification reduces the mineral content of adsorbents, resulting in improved acidic behaviours and a more hydrophilic nature of the adsorbent. It also improves surface charge, surface area, pore volume, lipophilic characteristics, and oxygen-based functional groups [161]. The outer surface positions of these groups are often referred to as adsorption sites. The concentration of oxygen at these points affects the adsorption capacity of the adsorbent [162]. Researchers have utilized acid-modified adsorbents for wastewater treatment [7,163].

Alkaline modification may involve the soaking of adsorbents in a solution of an alkali. Alkaline modification contributes to the development of positive charges on the adsorbent surface, thereby enhancing the adsorption of negative ions and the surface area overall. Also, alkaline modification aids porous slits in biomass, enhances surface basicity, and introduces oxygen-containing functional groups [164]. Widely utilized alkaline agents include sodium hydroxide (NaOH), potassium carbonate (K₂CO₃), and potassium hydroxide (KOH) [161]. Also, it can be utilized for the elimination of toxic compounds such as tetracycline, arsenic, atrazine, sodium, copper, and mercury. The KOH activation involves several steps such as dehydration, distortion of biomass, and lignocellulosic aromatization [165]. The NaOH and H₂SO₄ modification of lignocellulosic materials could represent a better chemical activation route [163,166,167].

6.2. Physical Activation

This is a cost-efficient, straightforward, and eco-friendly method, which involves utilization of gases and control of textural properties of adsorbents based on carbon and biomass. Sources such as ammonia (NH₃), carbon dioxide (CO₂), steam, air, helium (He), and gas mixtures could be engaged for carbon surface activation. Efforts have been made to enhance the surface area and pore volume of carbon materials via a range of physical treatment processes. Some of them are briefly discussed below.

Ball-milling is a less solvent-intensive technology that is gaining interest due to its eco-friendly and low-cost nature and could promote environmental sustainability. Ball-milling reduces particle sizes and enhances the surface functional groups of the biomass [159].

Gas activation involves the modification of biomass with several carrier gases along with CO₂ activation to produce an alternative mesoporous adsorbent [96]. Gas purging modification could improve the biomass surface area, pore volume, and alkali metal content in biomass [164].

The steam activation technique is a method for enhancing the textural features of carbon materials by improving the opening of more pores [166]. This technique involves the diffusion of steam into carbon material, thereby promoting pore formation. The mechanism of steam activation involves interaction between water vapour and carbon to generate hydrogen and carbon monoxide, with the co-production of methane and other gases [167]. There was an increase in the specific area, from 342 to 576 m²/g, and improved pore volume from 0.022 to 0.109 cm³, for biochar derived from tea waste [168]. The development of pore structure due to steam activation could be attributed to pore-clogging tar, enhancement of existing pores, and creation of new ones [167].

The air activating technique involves the utilization of air as an activating agent; oxygen is involved in this activation process due to its oxidizing properties. Air is commonly used for this purpose due to the high cost of oxygen [166]. Air oxidation has the potential to introduce new functional groups into carbon materials. Furthermore, this oxidizing process might result in a slight reduction in the surface area due to the new functional groups destructing the pores. The authors of [54] reported that air oxidation enriches the micropores of carbon materials and introduces O-bearing groups, resulting in reduced specific surface area. The researcher of study [103] carried out air activation on biochar at various temperatures and recorded important improvements in surface groups such as -OH, -C-O, and -C=O. This improvement can be attributed to the reaction involving oxygen and lignocellulosic components. They also examined the effect of activation duration on the process and noted initial improvement in the micropores within the initial 15 min, followed by enhanced mesopores content [103]. Conversely, a prolonged activation duration could result in micropore destruction, indicating the importance of selecting the appropriate activation time [168].

7. Conclusions

Pharmaceuticals are complex molecules that possess the potential to persist in aquatic environments and are resilient to conventional wastewater treatment techniques. For example, antibiotics are persistent due to largely ineffective treatment methods for wastewaters, resulting in a concentration range between 0 and 5320 ng/L. Furthermore, even at low concentrations, their long-term effects cannot be ignored, considering their impacts on human health and the survival of aquatic organisms. As new pharmaceuticals are being produced due to increased societal demands, it is necessary to implement effective strategies and regulations for the control of pharmaceutical releases into wastewaters, which consequently end up in surface waters. Simple yet advanced techniques for pharmaceuticals detection and standard protocols for the elimination of pharmaceuticals must be established.

Adsorption is an inexpensive treatment technique, which is easy to operate, eco-friendly, and effective for the removal of pharmaceuticals on an industrial scale. Adsorption has been acknowledged as a reliable process capable of competitively overcoming the main drawbacks associated with conventional wastewater treatment technologies, including being unattractive and uneconomical, requiring large spaces, and posing disposal challenges. The utilization of inexpensive agricultural wastes as an adsorbent precursor for efficient elimination of pharmaceuticals from surface water is a promising route, particularly for less wealthy countries. Low-cost adsorbents provide both environmental and economic benefits due to low environmental loads linked to the reusability, synthesis, cost, and recycling of adsorbents. Drawbacks might be linked to the utilization of low-cost adsorbents on a large scale. Furthermore, knowledge of the characteristics of the contaminants of interest and working conditions, such as pH, pressure, and temperature, must be considered. This review presented the adverse effect of pharmaceuticals on human health and the environment. Also, various modification methods were discussed, and adsorption efficiency of various low-cost adsorbents was summarized. Adsorption on low-cost adsorbents has the potential for water decontamination, which might be attributed to the presence of several functional groups on their surfaces. The wastes from agricultural activities were found to exhibit characteristics for the uptake of pharmaceutical contaminants. Several instrumentation techniques have been utilized to understand the internal and surface characteristics relevant to, and responsible for, improved adsorbent efficiency. Furthermore, surface modification might have immensely enhanced the adsorption efficiency of agro-food waste. Most studies related to the adsorption on modified adsorbents have been conducted on a laboratory scale. These modified adsorbents have not been subjected to complicated experimental settings to assess their efficiency. It is necessary to apply these adsorbents under the conditions that represent the actual contaminant environment. Application of adsorption technique with agricultural wastes could be utilized as an environmental cleaning technology and serve as an effective alternative to commercial adsorbents such as activated carbon.

Adsorption has been shown to be an effective process in the laboratory system. Its efficiency could be hampered by matrix effects in real surface waters. Natural organic matter (NOM), such as fulvic acid and humic acid, could occupy the active site via π-π and hydrophobic interactions, while inorganic ions such as Mg²⁺, Na⁺, Ca²⁺, and SO4²⁻ could precipitate with functional groups, hampering efficient pharmaceutical uptake. These competitive effects might influence the surface charge, block pores, and decrease adsorbent accessibility for porous materials. Future studies should evaluate adsorption at relevant environmental concentrations in the presence of ionic compositions and representative NOM using real water samples, alongside regeneration testing. This will aid in the accurate prediction of field-scale performance.

It is important to note that low-cost adsorbent involves the affordability of raw materials and the process of pre-treatment, which does not guarantee low management costs. Operational expenses such as chemical use, regeneration cycles, and adsorbent lifetime can influence total treatment costs. Also, low-cost precursors could require an extensive modification process to attain high performance, counterbalancing their initial viable advantage. Furthermore, life-cycle assessment and techno-economic assessment are essential to ascertain if an adsorbent is absolutely cost-effective in long-term application.

The fate of pharmaceuticals adsorbed on low-cost adsorbents is an important aspect of water treatment. Contaminants are bound to the adsorbent until it is disposed of or regenerated. Regeneration has aided in the recovery of adsorbent capacity and degraded bound pollutants. Chemical washing with an oxidizing agent could also be applied. Other approaches include co-incineration in cement kilns and hazardous waste incineration. It is important to select the proper disposal route, guided by adsorbent type, pollutant stability, and life cycle impact assessments.

The use of strong acids, bases, or oxidizing agents during the surface modification process could result in secondary waste and residual contaminants that could offset the sustainability merits of low-cost adsorbents. The importance of greener modification processes cannot be over-emphasized, such as low temperature activation, bio-based activating agents, and mechanochemical processing, alongside with reagent recovery and leaching tests to ensure environmental safety.

8. Future Direction

The continued advancement of pharmaceutical removal from surface waters using low-cost adsorbents requires a multi-pronged perspective. A greener modification technique should be prioritized to minimize the environmental footprint of adsorbent production. Also, adsorbent performance should be validated using water samples containing competing inorganic ions and natural organic matter to assure that laboratory results can be translated to field conditions. Furthermore, regeneration should be integrated into material design and the use of safe disposal options to reduce the re-release of pollutants. The application of a machine learning approach should be used to predict adsorbent–pollutant affinity, accelerate material screening, and guide functionalization. Life cycle and techno-economic assessments are required to distinguish the real sustainable and cost-effective low-cost adsorbent at the precursor stage. Addressing these approaches will help bridge the gap between laboratory-scale studies and scalable adsorption technologies for the elimination of pharmaceuticals.