Effective Hydrogel Surfaces for Adsorption of Pharmaceutical and Organic Pollutants—A Mini Review

Abstract

Organic and pharmaceutical pollution of water is a serious problem, particularly when it comes to drinking and groundwater. Although some evaluations indicate that these pollutants are unlikely to be at current exposure levels, they are often detected in aquatic systems and can be harmful to human health. Organic contaminants include hazardous micropollutants, aromatic phenols, pesticides, etc. Pharmaceutical contaminants are sulfamethoxazole, diclofenac, doxycycline, amoxicillin, trimethoprim, ciprofloxacin, norfloxacin, lipid regulators, nonsteroidal anti-inflammatory drugs (NSAIDs), hormones, antidepressants, etc. Hydrogel adsorbents’ distinct structural, chemical, and environmentally benign qualities make them a potential and successful option for environmental remediation, especially in wastewater treatment. In the search for clean water resources, they are an important instrument because of their reusability and capacity to be customized for certain contaminants, such as organic and pharmaceutical pollutants. This review focusses on the present state, adsorption sites and surfaces, different adsorption mechanisms, and the prospects and scope of improvement of effective hydrogels for eliminating dangerous aqueous organic and pharmaceutical contaminants. It offers a thorough summary of the area, highlighting its facets and potential paths forward.

1. Introduction

Pollutants are entities released into the environment that inflict damage on living beings and ecosystems. These are dangerous chemicals that pollute water in water systems, rendering them unhealthy and affecting the health of the environment. These substances can be inorganic or organic [1]. A large class of dangerous chemicals known as “organic pollutants (OPs)” are present in the environment, frequently in trace amounts, and represent serious threats to ecosystems and human health. They are constantly being added to water systems from a variety of home and commercial sources [2]. Pharmaceutical pollutants (PPs) are compounds created to improve human health and well-being. They are sometimes referred to as medications or biologically active pharmaceutical ingredients (APIs). These comprise a broad spectrum of drugs, including antidepressants, tranquillizers, analgesics, antibacterials, contraceptive tablets, and anticancer treatments [3,4]. PPs are classified as OPs; however, they differ primarily in their biological activity, main source, and extent. While not all OPs are medicines, all PPs are organic [5,6]. Due to their ubiquitous prevalence and possible negative effects on ecosystems and human health, micropollutants and pharmaceutical chemicals that contaminate water present serious environmental problems on a worldwide scale. Pesticides, medicines, and aromatic phenols are among the OPs that are constantly released into the environment by industrial processes (such as the printing, chemical, and cosmetic sectors) as well as domestic and medical waste. Important sources include petrol, medications, and personal care products (PCPs) from homes, factories, and medical facilities [7,8]. Common PPs that might evade conventional wastewater treatment methods and end up in processed water include diclofenac sodium (DFS), paracetamol, naproxen, acetylsalicylic acid, carbamazepine, and ibuprofen (IBU). Because of their tenacity, they are frequently seen in water sources [4,9]. A large class of very hazardous chemicals are known as OPs. Because they are persistent, they may readily bioaccumulate in the human body, causing immunological and reproductive system malfunction as well as other health problems like obesity, liver dysfunction, insulin resistance, diabetes, and cardiovascular illnesses. For example, endocrine-disrupting chemicals (EDCs) can contribute to the etiology of breast cancer in even minute doses. These contaminants have the potential to permanently harm the clean water system when they get into lakes, rivers, or streams [8,10]. Analgesics, tranquillizers, antidepressants, antibiotics, and contraceptive pills are examples of medications and biologically active pharmaceutical ingredients (APIs) that are intended to improve human health. Unconsumed parts are released into the environment through sewage systems and landfill leachate, as the human body may not fully digest the dosage that is taken. Additionally, unneeded and expired medications are frequently dumped straight into waterways, which contaminate groundwater [11]. Numerous techniques, such as desalination, precipitation, ion exchange, membrane separation, photocatalysis, flocculation, and electrocatalysis, are available to remove such contaminants [12,13]. Nevertheless, a lot of them have disadvantages, including being expensive, complicated to operate, or producing secondary pollutants [14]. Adsorption’s ease of use, high efficiency, and economic feasibility make it a very desirable method for getting rid of dangerous organic pollutants and PPs. Aqueous-phase components adhere to a solid surface in an exothermic process [15,16]. For the removal of organic and PPs from wastewater and other sources, a variety of adsorbents have been developed, including carbon nitride, biochar-based nanomaterials, chelating polymers, graft copolymers, hydrogels, ion exchange membranes, monoliths, resins and fibers, ion organic frameworks (MOFs), and more [17]. Soft, moist materials with a three-dimensional porous network structure are called hydrogels [18]. They have the amazing capacity to hold large amounts of water and expand while remaining semi-solid [19,20]. Water is an essential component that supports their shape, gives them mechanical qualities like elasticity, and aids in the dispersion of solutes [21]. The hydrogel matrix’s natural porosity offers a large surface area and pathways for contaminants to permeate and be trapped. A key factor in the adsorption properties of hydrogels is the existence of certain functional groups on their polymer chains. Through a variety of ways, these groups can interact with organic contaminants. Water molecules and perhaps organic contaminants can be captured more easily by hydrophilic groups such as hydroxyl (-OH), amine (-NH₂), carboxyl (-COOH), and amide (-CONH-), which can create strong hydrogen bonds with them [22,23]. Aqueous hazardous contaminants can be effectively removed by functional hydrogel surfaces because of their physicochemical characteristics, affordability, ease of use and manufacture, high specific surface area and porosity, and simple reusability. By means of electrostatic and hydrogen (H)-bonding interactions, they demonstrate an exceptional adsorption capability for a range of pollutants, including poisons and dangerous organic compounds. These resources are plentiful, reasonably priced, non-toxic, and environmentally beneficial [24,25]. For example, Kunori et al. developed a new semi-interpenetrating polymer network (semi-IPN) hydrogel using polyethyleneimine/poly(N-isopropylacrylamide) (PEI/poly (NIPA)). As a high-performance adsorbent, its main function was to recover individual organic chemicals and cleanse wastewater that included many organic compounds. In particular, p-nitrophenol and 4-iodoaniline, which were used as model adsorbates, were effectively removed (water purified) and mutually separated (recovered) by the hydrogel [26]. The objective of this review is to explore the hydrogel surface properties suitable for adsorption. The adsorption mechanism of organic pollutants and PPs on the effective surfaces of hydrogels is also discussed.

2. Hydrogel Surfaces for Adsorption

Hydrogel surfaces are essential due to their ability to remove impurities from aqueous solutions as efficient adsorbents. The internal structure of hydrogels and their surfaces help bind different types of contaminants [27]. The morphology of hydrogel surfaces can vary, showing up as thin films with uniform textures, smooth or porous surfaces, or rough spherical beads. Their adsorption capacities depend heavily on these surface properties as well as the interior porous network. For instance, the exterior surface of hydrogel beads can be extremely rough and feature numerous holes, which increases adsorption [28]. Functional groups including carboxyl (-COOH), amine (-NH), hydroxyl (-OH), and imidazole groups are abundant on the surfaces of hydrogels. Their ability to adsorb is largely dependent on these groups [29,30]. The capacity of hydrogels to absorb and retain water and contaminants, as well as their distinctive three-dimensional, crosslinked porous network structure, allow them to have substantial confinement effects on pollutants and solutions. Pollutant removal efficiency and selectivity are determined by these effects. PPs and organic dyes have different molecular adsorption processes because of differences in their chemical structures, functional groups, charge distributions, and interactions with adsorbent surfaces. There is no big difference in the adsorption mechanism between Ops and PPs in terms of hydrogel surfaces [31]. The following are some of the main mechanisms used to confine pollutants through separation. (i) Size exclusion: This is a process in which the hydrogel’s pore size regulates the flow of molecules, successfully collecting bigger pollutants whose hydrated ionic diameters are greater than the pore size [32]. Predicting the efficacy of size exclusion requires knowledge about molecular properties such as size, shape, and molecular weight [33]. (ii) Electrostatic interactions: The contaminants in the solution and the charges on the surface of the hydrogel are important factors. The ability of ionic or zwitterionic hydrogels to either attract or repel charged particles improves the effectiveness of separation. While attraction makes it easier to separate oppositely charged species, repulsion happens when the hydrogel and particles have the same charge, inhibiting their close proximity [34]. (iii) Non-sieving processes: Electrostatic repulsion between ionic molecules and fixed charges in the membrane matrix causes the membrane to selectively limit ionic transport, a phenomenon known as Donnan exclusion. Additionally, ion interactions with bound electric charges at interfaces between dielectrically different media lead to dielectric exclusion [35]. Hydrogel adsorbents’ efficacy and durability for the removal of pollutants are greatly improved by dynamic covalent bonding and self-healing properties. The materials’ reprocessability, reusability, and repairability are enhanced by these characteristics [36]. The addition of multifunctionality to hydrogels is mostly dependent on dynamic covalent bonds, particularly disulfide-containing crosslinkers. Because of these bonds’ robustness (around 60 kcal mol⁻¹ bond dissociation energy) and dynamicity (quick thiol-disulfide exchange processes), smart materials with adaptability and repairability may be designed. The reformation of disulphide crosslinks at sliced surfaces via disulphide exchange reactions allows these hydrogels to self-heal. Heating (for example, at 80 °C) can speed up this process and enable the hydrogel to self-heal in a matter of minutes (for example, five minutes). Long-term uses depend on the adsorbent’s endurance, which is directly influenced by its self-healing capacity. Retained stretchability even after healing is proof that it enables the hydrogel to tolerate mechanical stress and preserve its structural integrity. This repairability guarantees that the hydrogel’s adsorption capacity will not be significantly reduced even after several adsorption–desorption cycles. For instance, there was minimal variation in the adsorption capacity (e.g., 2.8, 2.5, and 2.7 mg/g for each run) throughout several adsorption–desorption cycles [37,38]. Advanced materials known as stimuli-responsive hydrogels have the ability to alter their chemical and physical characteristics in response to external stimuli, such as enzymes, pH, temperature, light, and ionic strength. Because of their versatility, they are very useful in environmental applications, particularly in wastewater treatment, where they effectively eliminate organic and pharmaceutical contaminants [39]. OPs are mostly removed using stimuli-responsive hydrogels using a variety of adsorption techniques and stimulus-induced modifications to increase removal effectiveness. A major factor in the effectiveness of their removal is the tendency of non-polar organic molecules to attach themselves to the hydrophobic areas of the hydrogel matrix. For instance, because of their temperature-responsive phase transitions, which change the hydrophobicity of the hydrogel surface, poly(N-isopropylacrylamide) (PNIPAM) hydrogels may efficiently absorb hydrophobic pollutants. In reaction to stimuli such as temperature or pH, hydrogels may expand or contract, changing their surface area and pore size and making it easier for pollutants to be absorbed. For example, PNIPAM-based temperature-responsive hydrogels show a dramatic volume phase shift at their lower critical solution temperature (LCST), which increases their surface area and improves OP adsorption [40]. Hydrogels containing functional groups can interact with charged organic contaminants through ion exchange and electrostatic forces, increasing their adsorption capability, but they are more frequently mentioned in relation to heavy metals. When exposed to UV light, light-responsive hydrogels may photodegrade OPs, providing an energy-efficient removal technique. This action may be amplified by adding photocatalytic elements to the hydrogel matrix, which enables simultaneous adsorption and degradation [39]. Photonic crystal hydrogels have great potential for sensing applications because they combine the structural color of photonic crystals with the characteristics of hydrogels. Enhancing their manufacture and comprehending their sensitivity to different stimuli have been the main goals in recent studies. Nonetheless, issues with mechanical characteristics and repeatability have been noted. Moulding techniques have been created to create highly reproducible photonic crystal hydrogels. This technique transfers nanostructures onto the hydrogel surface by using a silicon wafer containing a monolayer of self-assembled nanoparticles as a mould. Different solvents cause reversible color changes in photonic crystal hydrogels. This is because the hydrogel’s volume changes, changing the photonic crystal’s periodicity. Colors are red-shifted while swelling and blue-shifted when contracting. The ratio of monomer to crosslinker affects the mechanical characteristics of the hydrogel, including its stiffness and elasticity. While a lower ratio produces a stiffer hydrogel, a greater ratio indicates fewer crosslinks, which increases flexibility. Traditional techniques for creating photonic crystal hydrogels, especially those that rely on colloidal particle self-assembly, sometimes have trouble with repeatability and homogeneity on a wide scale. Pattern fidelity may be limited by problems such as domain borders, cracking, and multilayer areas [41]. One of the main mechanisms for adsorption on hydrogel surfaces is electrostatic interactions. Functional groups on the hydrogel surface can protonate or deprotonate depending on the pH, generating charged sites that attract oppositely charged contaminants (e.g., metal ions and dyes). For example, amino groups can protonate to generate -NH₃⁺ groups in acidic environments, which attract negatively charged contaminants such as Cr⁶⁺. On the other hand, the negatively charged COO- groups are deprotonated by the -COOH groups in cationic pollutants, which draws in the positively charged ions shown in Figure 1.

Ion exchange, in which ions on the hydrogel surface are swapped out for polluting ions from the solution, is another significant process. This is especially important for the elimination of heavy metal ions [42]. Additionally, chelating interactions, hydrophobic contacts, coordination bonds, and hydrogen bonds—often in combination—can result in adsorption [30]. Table 1 presents the hydrogels used for the removal of pharmaceutical and organic pollutants. Hydrogels commonly used in water/wastewater treatment are classified into three main types based on their shape and physicochemical properties. (i) Hydrogel beads—These round beads, which frequently have rough surfaces, are frequently employed to remove dyes and heavy metal ions. Beads made of graphene oxide (GO)/PEI, chitosan, and cellulose are a few examples [33,43]. (ii) Hydrogel films—These thin, uniformly textured sheets are employed in filtering membranes and other applications. Agarose/CD hydrogel films and carbon dot hybrid films with chitosan are two examples [44]. (iii) Hydrogel nanocomposites—These include clay mineral-loaded, soy protein-based, and hyperbranched polymer/GO nano-composites, which mix hydrogels with nanoparticles to improve adsorption efficiency and qualities like mechanical strength [45].

Table 1. Comparison of different hydrogels for pharmaceutical and organic pollutant adsorption.

Types of Hydrogels	Adsorbate	Adsorption Efficiency	Limitation	References
Graphene hydrogels	Ofloxacin	105 mg/g gel	Lack of fast, cheap, and secure monitoring	[46]
Lignin sulfonate (LS)/acrylamide (AAm)	diclofenac	50 mg/g gel	The inherent ‘hydrophobicity’ of lignin	[47]
Polyacrylamide-polyacrylic acid (PAA-PAM)	ciprofloxacin	1401.53 mg/g gel	Aggregation of hydrogel reduces efficiency and recoverability	[48]
Polyvinyl alcohol-acrylic acid (PVA-AMA)	Streptomycin	93.5 mg/g gel	Sensitivity to changes in ionic strength	[49]
Kappa carrageenan hydrogel	Oxytetracycline	2.567 mg/g gels	Lower gel strength	[50]
Polyethyleneimine/poly(N-isopropylacrylamide) semi-IPN hydrogel	hydrophobic organic pollutants	p-nitro phenol—68.8 mmol/kg-dry gel 4-iodoaniline—28.7 mmol/kg-dry gel	Limited selectivity	[26]
β-cyclodextrin-polyacrylamide hydrogel	Organic Micropollutants	Phenolphthalein—34.97 mg/g Bisphenol A—29.59 mg/g propranolol hydrochloride—8.45 mg/g 2-napthol—55.56 mg/g	Costly and energy-consuming thermal regeneration	[51]
Polyvinyl alcohol/porous carbon composite hydrogels	Organic Pollutants	Congo red—27.39 mg/g Ibuprofen—15.48 mg/g Doxycycline hydrochloride—17.07 mg/g	Fragility of the hydrogel	[52]
Poly (N-isopropylacrylamide)-sodium carboxymethyl cellulose hydrogel	Organic nanoplastics	199.64 mg/g	Mechanical fragility limits industrial applicability	[53]
Laccase-cellulose-DNA hydrogels	Micro-organic pollutants	The degradation rate of Flu (0.36 μg/h), 1-MFlu (0.38 μg/h) and 3-NFlu (0.30 μg/h)	Limited surface area and weak interaction forces	[54]
Immobilized laccase/chitosan/sodium alginate hydrogel	Organic Pollutant Bisphenol A	34.92 µmol/L	Inadequate mechanical strength, poor stability, and low solubility	[55]
Graphite carbon nitride/calcium alginate	Rifampicin	The removal rate of 150 mg/L rifampicin	Limited selectivity hinders effectiveness	[56]

Table 1. Comparison of different hydrogels for pharmaceutical and organic pollutant adsorption.

Types of Hydrogels	Adsorbate	Adsorption Efficiency	Limitation	References
Graphene hydrogels	Ofloxacin	105 mg/g gel	Lack of fast, cheap, and secure monitoring	[46]
Lignin sulfonate (LS)/acrylamide (AAm)	diclofenac	50 mg/g gel	The inherent ‘hydrophobicity’ of lignin	[47]
Polyacrylamide-polyacrylic acid (PAA-PAM)	ciprofloxacin	1401.53 mg/g gel	Aggregation of hydrogel reduces efficiency and recoverability	[48]
Polyvinyl alcohol-acrylic acid (PVA-AMA)	Streptomycin	93.5 mg/g gel	Sensitivity to changes in ionic strength	[49]
Kappa carrageenan hydrogel	Oxytetracycline	2.567 mg/g gels	Lower gel strength	[50]
Polyethyleneimine/poly(N-isopropylacrylamide) semi-IPN hydrogel	hydrophobic organic pollutants	p-nitro phenol—68.8 mmol/kg-dry gel 4-iodoaniline—28.7 mmol/kg-dry gel	Limited selectivity	[26]
β-cyclodextrin-polyacrylamide hydrogel	Organic Micropollutants	Phenolphthalein—34.97 mg/g Bisphenol A—29.59 mg/g propranolol hydrochloride—8.45 mg/g 2-napthol—55.56 mg/g	Costly and energy-consuming thermal regeneration	[51]
Polyvinyl alcohol/porous carbon composite hydrogels	Organic Pollutants	Congo red—27.39 mg/g Ibuprofen—15.48 mg/g Doxycycline hydrochloride—17.07 mg/g	Fragility of the hydrogel	[52]
Poly (N-isopropylacrylamide)-sodium carboxymethyl cellulose hydrogel	Organic nanoplastics	199.64 mg/g	Mechanical fragility limits industrial applicability	[53]
Laccase-cellulose-DNA hydrogels	Micro-organic pollutants	The degradation rate of Flu (0.36 μg/h), 1-MFlu (0.38 μg/h) and 3-NFlu (0.30 μg/h)	Limited surface area and weak interaction forces	[54]
Immobilized laccase/chitosan/sodium alginate hydrogel	Organic Pollutant Bisphenol A	34.92 µmol/L	Inadequate mechanical strength, poor stability, and low solubility	[55]
Graphite carbon nitride/calcium alginate	Rifampicin	The removal rate of 150 mg/L rifampicin	Limited selectivity hinders effectiveness	[56]

Techniques for surface analysis are essential for comprehending the characteristics and behavior of hydrogels, particularly when they are used for processes like the adsorption of OPs. Fourier Transform Infrared (FTIR) spectroscopy, Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM), and Scanning Electron Microscopy with Energy Dispersive X-ray (SEM/EDX) are typically used in studies. The functional groups involved in the adsorption process are identified using FTIR spectroscopy, which is also utilized to track changes in the hydrogel’s chemical structure both before and after adsorption [57]. For describing the surface morphology and dimensions of a variety of materials, including hydrogels and their nanocomposites, Transmission Electron Microscopy (TEM) is an essential method. TEM offers comprehensive information on the distribution of integrated components and the structural characteristics of hydrogel surfaces. For instance, Figure 2 displays the results of a TEM investigation performed by Selcan Erdem et al. on pure halloysite nanotubes, pure chitosan nanoparticles, and synthesized halloysite–chitosan nanocomposites. The halloysite-based nanocomposites’ morphology is described by TEM, which also highlights the modifications brought about by the addition of chitosan. TEM photos of pure halloysite nanotubes show the nanotubular structure clearly. According to the literature, halloysite nanotubes usually range in length from 500 nm to 1.2 µm and have a diameter of less than 100 nm. Natural nanotubes with diameters ranging from 50 nm to 200 nm and lengths between 300 nm and 800 nm were observed in this investigation. Agglomeration was not seen in TEM images of pure chitosan nanoparticles made with a 0.25% TPP cross-linking agent. These chitosan nanoparticles were found to be less than 50 nm in size. TEM pictures of halloysite/chitosan nanocomposites (at a mass ratio of 3:1) showed that the chitosan layer was not always easily distinguishable due to its lower electron density. On the surface of the halloysite nanotubes, chitosan was observed to develop a rough texture. Following chitosan coating, the smooth, recognizable tubular structure of the halloysite nanotubes became hazy, uneven, and rough, and the coating took the form of light grey clusters [58].

The most important descriptive tools, Energy-dispersive X-ray Spectroscopy (EDX/EDS) and Scanning Electron Microscopy (SEM), are essential methods for describing the morphology, microstructure, and elemental composition of hydrogels, particularly after they have been employed for adsorption [59]. Prior to adsorption, SEM pictures show that the hydrogel structure is very porous. A significant reduction in hydrogel pores is seen upon adsorption. The presence of elements on the pollutant-loaded hydrogel was verified by EDX spectra, which showed that the strength of the element’s peak, which was absent in the undamaged hydrogel, was proportionate to its concentration.

Aleksandar Zdravkovi et al. synthesized poly (NIPAM-co-MAA) hydrogels that exhibit a porous structure with varying pore sizes (5–30 µm) when swollen and lyophilized. The transportation of simulated metal solutions and the binding of contaminants to functional groups in the hydrogel seen in Figure 3 depend heavily on this porous character. In comparison to hydrogels that have just inflated in water, SEM micrographs show notable alterations in the microstructure of hydrogels following their absorption of pollution particles. This suggests that the functional groups on the hydrogel surface are bound by the buildup of contaminants [60]. The presence of elements, particularly heavy metals, in the sorbent hydrogels is ascertained by EDX spectroscopy (Figure 4). Following the sorption process, it is verified that every ingredient is present in the hydrogel structure. The distinctive elements are represented by certain energy peaks in the EDX spectra. For example, Cr exhibits peaks at 0.573 and 5.411 keV. Hydrogel-specific elements, such as oxygen (O) at 0.525 keV, nitrogen (N) at 0.392 keV, and carbon (C) at 0.277 keV, are also visible in the EDX spectra [60].

AFM is a potent surface analysis method that is used to examine the compositional and topographical changes of hydrogels, especially before and after the adsorption of metal ions. AFM was used by Milosavljevic et al. to examine hydrogels of chitosan, itaconic acid, and methacrylic acid (Ch/IA/MAA). They used AFM to monitor changes in hydrogel surface topography and to identify surface composition changes using phase imaging, as seen in Figure 5. This enables researchers to comprehend how the adsorption process affects the hydrogel surface. Therefore, AFM shows surface changes and offers important insights into the physical changes on the hydrogel surface during metal ion adsorption [57].

3. Adsorption of Pharmaceutical Pollutants

One important class of emerging contaminants (ECs) that can endanger ecosystems and human health even at extremely low concentrations is pharmaceutical contaminants (PCs). These substances do not readily break down and have a stable structure, which makes them persistent in the environment. They are categorized into a number of types according to their mode of action or therapeutic uses, as shown in Figure 6 [61].

Analgesics and anti-inflammatory drugs, which are used to treat pain or lower a fever, are two of the main sources of pharmaceutical contaminants (PCs) in wastewater. They are categorized according to how they work, such as COX-2 inhibitors, opioids (morphine), cannabis, paracetamol (acetaminophen), and non-steroidal anti-inflammatory medications (NSAIDs). Ibuprofen, paracetamol, and diclofenac are typical examples. NSAIDs have long-lasting ecotoxic impacts on biotic elements of ecosystems, even at very low concentrations [62,63]. Antibiotics are antimicrobial substances that either eradicate germs or prevent the growth of bacteria. They are used to cure infectious illnesses, protect human health, and stimulate the growth of animals. They are categorized as macrolides, tetracyclines, cephalosporins, quinolones, lincomycins, penicillins, sulfonamides, and tetracyclines according to their chemical structure and mode of action. Antibiotic resistance genes (ARGs) are a global public health problem that can evolve and spread as a result of antibiotics in the environment [64,65]. Neuroactive pharmacological substances known as antidepressants are used to treat conditions involving chronic pain, addiction, schizophrenia, and depression. They are highly bioavailable, easily absorbed, and mostly basic in nature. These include tricyclic antidepressants (TCAs), tetracyclic antidepressants (TeCAs), monoamine oxidase inhibitors (MAOIs), selective serotonin reuptake inhibitors (SSRIs), serotonin–norepinephrine reuptake inhibitors (SNRIs), serotonin modulator and stimulators (SMSs), serotonin antagonist and reuptake inhibitors (SARIs), norepinephrine reuptake inhibitors (NRIs), norepinephrine reuptake inhibitors (NDRIs), tricyclic antidepressants (TCAs), and tetracyclic antidepressants (TeCAs) [66,67]. The inherent ability of pharmaceutical products (PPs) to cause a wide range of physiological effects in humans and other creatures makes them a unique class of newly discovered environmental toxins that present serious dangers. Their pervasiveness throughout a range of environmental matrices, including aquatic systems, poses serious risks to human health and biodiversity [11]. PPs are present around the world in a variety of environmental contexts, such as sediments, groundwater, and surface water. They are ‘pseudo-persistent’ chemicals because of their continual discharge, even though their concentrations are very low, frequently with several micrograms or nanograms per liter [68]. Many PPs, like carbamazepine, have a history of poor degradation in wastewater treatment procedures, making them extremely persistent in the environment [69]. Conventional wastewater treatment systems are unable to totally eliminate them due to their resistance [70]. Leaching from landfills, wastewater treatment facilities, and post-use sewage treatment facilities are the main ways that PPs reach the environment. Additionally, they can contaminate drinking water and groundwater through agricultural runoff from processed sludge used as fertilizer [71]. PPs have the ability to cause physiological abnormalities and disrupt biological processes in a variety of creatures. Many PPs can have negative consequences even at modest dosages [72]. Their potential to disrupt the endocrine system, leading to imbalances and undesirable consequences, is a significant worry. A broad range of substances, including certain medications known as endocrine disruptors, can have an impact on the health and offspring of an organism [73]. A single PP may not have any discernible toxicological consequences, but because of their additive and synergistic actions, coupled PPs can produce noticeable ecotoxicity even at low doses. Daphnia magna, for instance, was more affected by exposure to clofibric acid and carbamazepine together than by either substance alone [74]. Antibiotics in the environment have the potential to cause microbial populations to become widely resistant to them, which makes treating microbial infections and illnesses extremely difficult [75]. Marine species’ organs, tissues, and plasma can all contain PPs [18]. Because lipophilic chemicals are delivered to people through dietary intake, this bioaccumulation can take place across the food web [19]. Tonalide and galaxolide in fish samples are two examples [76]. In particular, PPs are having an impact on certain species. Diclofenac, for instance, has been demonstrated to seriously alter fish immunological parameters and jeopardize the integrity of the kidneys and gills. Erythromycin can alter the structure and function of microorganisms [77]. In mussels, exposure to ibuprofen can cause serious disruption of the digestive membranes [78]. Particularly in agricultural systems that use treated wastewater for irrigation, PPs can build up in plant tissues [79]. There are several ways that hydrogel and pharmacological contaminants might interact. First, the elimination of drug compounds is mostly driven by electron donor–acceptor (EDA) interactions. This encompasses (i) π–π interactions, which occur between the hydrogel’s π-electron-depleted regions and the drug molecules’ π-electron systems; and (ii) n–π interactions, which include electron-donating compounds from the therapeutic molecules that interact with the hydrogel’s π-electron systems, such as secondary amines, -OH, OR, and alkyl groups. Secondly, hydrogen bonding can occur between the drug molecules’ comparable functional groups and the other hydrophilic oxygen-containing moieties (-OH, COOH, and C=O) dispersed across the hydrogel surface. Hydrophilic functional groups of the drug molecules and water molecules trapped in the hydrogel can also form hydrogen bonds [80]. Third, hydrophobic interactions are important, particularly for substances that are hydrophobic. When labile oxygen-containing groups are eliminated during the hydrogel’s creation, the hydrophobicity is increased, improving the hydrophobic medicine’s ability to self-assemble and adsorb [81]. Fourth, although less common, electrostatic forces can nevertheless play a role in interaction, particularly in natural pH environments in which acidic medications may experience electrostatic repulsion. These interactions may be influenced by the hydrogel’s surface charge, which turns negative at higher pH levels as a result of the de-protonation of carboxyl and hydroxyl groups. Fifth, the hydrogels’ porous three-dimensional network structure makes it easier for drug molecules to enter the network, which speeds up the adsorption kinetics. By creating transport nanochannels and promoting hydrogen bonding, the buried water in the hydrogels also has a positive effect [82]. According to Nadia Umbreen et al., self-assembled three-dimensional hydrogels based on reduced graphene oxide (RGO) interact with pharmaceutical contaminants through a variety of plausible processes and synergistic effects, allowing for their effective removal from the aqueous solutions depicted in Figure 7. The capacity of the hydrogels to function as super-adsorbents for a variety of pollutants depends on these interactions. The main processes controlling how RGO-based hydrogels interact with and absorb pharmaceutical medications such as diclofenac (DFC), ibuprofen (IBP), and naproxen (NPX) are shown. The shape of hydrogel surfaces, the adsorption process of pollutants on the surface, and the disposal of PPs are all depicted in [83].

The hydrogels are very effective and make it simple to remove pharmacological substances from aqueous solutions. RGO-based hydrogels are regarded as safe for the environment.

In spiking genuine environmental aqueous samples (wastewater, river water, and tap water), the hydrogels demonstrated good removal efficiency, suggesting that they may be used for wastewater purification. Even though hydrogels exhibit high recyclability, more research is required to increase the regenerability of wasted hydrogels to guarantee wastewater treatment systems’ long-term sustainability advantages. Higher salt concentrations reduced the removal effectiveness of several medications (NPX and IBP), indicating that settings with high ionic strength may limit the number of binding sites accessible for medicinal pharmaceuticals.

Mariana Chelu et al. created a hydrogel using chitosan, polyethylene glycol 4000, and xanthan gum (CPX) to effectively adsorb the medication diclofenac sodium (DCF). This process mostly relies on hydrogen bonds and electrostatic interactions, with a small amount coming from Van der Waals forces, as seen in Figure 8 The hydrophilic and hydrophobic qualities of DCF itself as well as the hydrogel’s constituents have an impact on the procedure. There are three types of interactions. In electrostatic interactions, the positively charged chitosan (because of its NH₃⁺ groups) and the negatively charged xanthan gum (because of its COO- groups) and PEG interact electrostatically to create a complex in the CPX hydrogel. The hydrogel structure is strengthened, and its viscosity is increased by this complexation. Likewise, DCF, a polar molecule, contains both hydrophilic functional groups (phenolic and carboxylic acid groups) and hydrophobic functional groups (chloro substituents). A major factor in DCF’s adsorption is the electrostatic interactions between the hydrogel’s charged constituents. The experimental data is well-fitted by the Langmuir model, which postulates monolayer adsorption on homogenous and independent sites, suggesting that electrostatic interactions are the main driving factor. During hydrogen bonding, PEG may connect with water molecules because of its hydrophilic characteristics, which include its hydroxyl (-OH) groups. In addition, xanthan gum has a lot of hydrophilic functional groups, such as carboxyl (-COOH) and hydroxyl (-OH) groups, which help make it soluble in water. Because of its amino and hydroxyl groups, chitosan may also form hydrogen bonds with water molecules. The adsorption process is aided by these hydrogen bonding interactions between the hydrogel and DCF as well as inside the hydrogel network. Van der Waals forces also play a role in the adsorption of DCF by the hydrogel, even if hydrogen bonds and electrostatic interactions are the primary processes. Additionally, chitosan has hydrophobic acetyl and amino groups that can engage in Van der Waals interactions with non-polar molecules [84]. Therefore, more functional groups, a strong network structure, and a high porosity of the hydrogels yield a better performance in the removal of pharmaceutical pollutants.

A green, easy, inexpensive, and ecological aqueous solution approach is used to manufacture hydrogels. Adsorbents based on hydrogels, especially those derived from natural biopolymers like xanthan gum and chitosan, are renowned for being environmentally benign, non-toxic, and biodegradable. Hydrogels and other adsorption-based techniques are appealing for treating wastewater because of their great efficacy. The CPX hydrogel is regarded as being reasonably cost-effective. According to the findings from the kinetic investigation, the CPX hydrogel offers a great deal of promise for usage as a water cleaning agent in environmental applications. The CPX hydrogel gradually absorbs the diclofenac sodium, and after 350 min, the elimination efficiency reaches its peak. The dry CPX hydrogel’s low moisture content (5.22 ± 0.43%) accounts for its decreased flexibility and hardness. The regeneration and reusability of this particular CPX hydrogel, which are frequent factors for adsorbents, are not specifically covered in the material that was supplied.

Therefore, the CPX hydrogel’s green synthesis and excellent adsorption characteristics make it a viable option for pharmaceutical contaminant removal.

4. Adsorption of Organic Pollutants

Hydrogels’ exceptional swelling behavior, high porosity, and abundance of functional groups (such as hydroxyl, carboxyl, and amine groups) facilitate effective interactions with a wide range of organic contaminants. These interactions are almost the same as those with PPs. In particular, functional group modifications on the hydrogel backbone allow for targeted adsorption by enhancing its affinity for specific classes of pollutants, such as aromatic dyes. The incorporation of nanomaterials, such as graphene oxide (GO), metal–organic frameworks (MOFs), and supramolecular hosts like pillararene, further improves the selectivity and capacity of hydrogels through synergistic effects and the creation of novel adsorption sites [53,82]. Hydrogen bonding is an intermolecular or intramolecular interaction involving a covalently bonded H atom on a donor atom (X) and an electronegative acceptor atom (Y) with lone pairs, commonly N, O, or F. Though weaker than covalent or ionic bonds, it surpasses van der Waals forces and plays a role in dye removal from water through combined hydrogen bonding, electrostatic, and van der Waals interactions. For instance, dialdehyde carboxymethyl cellulose–gelatin hydrogels achieve high dye removal efficiencies (up to 96.5% for Rhodamine B) at a pH of 6, facilitated by these interactions. Similarly, cellulose/alginate hydrogels adsorb dyes like Methylene blue via hydrogen bonds (O-H⋯N) and electrostatic forces, leveraging hydroxyl and carboxyl groups [85]. Hydrogels modified with graphene preferentially adsorb aromatic dyes like Congo red, MB, and methyl orange, due to the π-electrons in graphene’s honeycomb lattice. Mittal et al. developed a graphene oxide-based hydrogel using chitosan and carboxymethyl cellulose for dye removal, achieving maximum adsorption capacities of 655.98 mg/g for MB and 404.52 mg/g for MO [86]. The adsorption kinetics of hydrogels typically conform to pseudo-second-order models, indicating chemisorption processes governed by electron sharing or exchange between the hydrogel and the adsorbate. Adsorption isotherms commonly fit the Langmuir model, suggesting monolayer adsorption on homogeneous surfaces. Various studies have reported high adsorption capacities for OPs. For example, a supramolecular hydrogel based on pillar [5] arene exhibited an outstanding adsorption capacity of 1818 mg/g for Eriochrome Black T (EBT), significantly surpassing that of conventional activated carbon [87]. Nishil et al. synthesized cellulose nanocrystal and alginate (CNG-ALG) hydrogels with a superior adsorption capacity, which can adsorb methylene blue from aqueous solution with a maximum capacity of 256.41 mg/g, and the optimum removal efficiency can reach up to 97% [88]. The author showed that the adsorption followed pseudo-second-order kinetics and the intra-particle diffusion mechanism (Figure 9). The equilibrium adsorption data fitted well to the Langmuir adsorption isotherm.

Alginate hydrogel beads’ adsorption capacities are greatly improved by the addition of CNCs because of the increased surface area and binding sites. Approximately 97% of the dye is removed by the highly recyclable CNC-ALG hydrogel beads, even after five adsorption–desorption cycles. Following regeneration, they regained nearly all of their adsorption capability. This reusability is essential for real-world wastewater treatment uses. These hydrogels are composed of sustainable nanomaterials, specifically cellulose nanocrystals from pulp fibres and alginate from brown seaweed. Their application lessens reliance on adsorbents, such as activated carbon, which, when activated, produce greenhouse gases.

The adsorption capacity of freeze-dried hydrogel beads was lower than that of moist hydrogel beads. This is explained by the fact that drying reduces porosity, which forms a structural barrier that restricts dye molecule transport to the adsorption site. In comparison to wet beads, dried alginate beads are known to have reduced porosity and slower metal ion adsorption rates. The greater crystallinity of CNCs results in a relatively lower swelling ratio for CNC-ALG beads compared to pure ALG hydrogel beads.

Similarly, for anionic dyes such as eosin B (EB) and Alizarin red S (ARS), Banerjee et al. reported cross-linked poly(acrylamide) (CPAam) and poly (2-hydroxyethyl methacrylate) (CPHEMA) hydrogels, which show excellent removal efficiencies. After two days of incubation, the clearance efficiency for EB dye might reach 99.8%. The removal efficiency for ARS ranged from 74% to 89.2%, which was similarly excellent. The anionic dye molecules and the hydrogel’s cationic cross-linking sites interact electrostatically to produce most of the adsorption. Particularly for crystal violet (CV) dye adsorption, other interactions such as H-bonding and π–π stacking also play a role. Maximum adsorption capacities for eosin B are over 100 mg g⁻¹, demonstrating the hydrogels’ great ability to adsorb anionic dyes. Compared to several other documented adsorbents, this is significantly greater. Because the hydrogels selectively adsorb anionic dyes, they hold promise for the separation of wastewater containing anionic-cationic color combinations. For up to three consecutive dye adsorption–desorption cycles, the hydrogels retain a recycling efficiency of over 80% and exhibit exceptional recyclability and regeneration. After desorption, their surface shape, including their macroporous structures, can be restored, enabling further activity. Their removal efficacy for cationic dyes is much lower than that for anionic dyes because of the electrostatic repulsion that exists between cationic dyes and the hydrogel’s cationic sites. Process optimization can be difficult due to the adsorption mechanism’s complexity, which includes many diffusion stages (bulk, film, and intraparticle diffusion). Although the hydrogels absorb the dye quickly at first, it takes them around 24 h to attain an equilibrium and saturate all the adsorbing sites [89]. The process by which dyes adhere to these hydrogels aligns with a pseudo-second-order kinetic model. Insights into the mechanism are detailed through intraparticle diffusion and the Boyd model. The highest equilibrium adsorption capacity (q_m) for eosin B (EB) fits both the Langmuir and the Freundlich isotherm models (Figure 10).

Hydrogels can adsorb a broad spectrum of OPs, including synthetic dyes (e.g., methylene blue, rhodamine B, and Congo red), and emerging contaminants such as endocrine-disrupting compounds and cosmetic residues. Several environmental parameters, including pH, temperature, pollutant concentration, and contact time, influence the adsorption efficiency. Generally, slightly alkaline conditions favor the adsorption of cationic dyes due to the increased deprotonation of carboxyl groups, which enhances electrostatic interactions. Furthermore, the reusability and regeneration of hydrogels remain important considerations for real-world applications. Some hydrogel systems have shown high recyclability, maintaining substantial adsorption efficiency over multiple cycles, although performance may decline depending on the regeneration method employed. Although there is a noticeable decrease in their capacity when dried, CNC-ALG hydrogel beads offer a promising, environmentally friendly, and extremely efficient option for the removal of organic dyes from wastewater. They also offer notable benefits in terms of adsorption performance and reusability.

5. Challenges and Scope of Improvements

Conventional adsorbents frequently perform poorly in wastewater purification because of their limited adsorption capacity and challenges in recovery or recyclability. Real water samples, such as river water or wastewater, may have complex matrices that might influence how drug molecules and hydrogels interact [90]. The hydrogel’s adsorption process ultimately reaches a plateau, suggesting that the interstitial spaces inside the hydrogels are saturated. This suggests that the capacity to absorb pollutants is limited [83]. Hydrogels have several drawbacks that affect their effectiveness, durability, and usefulness even though they are promising materials for the adsorption and degradation of OPs. These drawbacks include problems with light penetration, reusability, and the requirement for the further treatment of contaminants that have been adsorbed. The photodegradation activity of many hydrogel composites decreases after several cycles of usage [91]. Although adsorption is a versatile and economical physical technique for eliminating a range of contaminants, the adsorbed pollutants must be desorbed and treated afterwards. This requires an additional stage for the entire water purification procedure. Extreme basicity can cause competition for adsorption sites between the pollutant and hydroxide ions, which can lower photocatalytic performance [92]. While the goal of hydrogels is to break down pollutants into less dangerous molecules, it is important to verify if full degradation has been achieved and evaluate any byproducts’ possible toxicity because certain intermediate products could be more deadly than the original pollutants [93]. To mitigate the current limitations, new types of innovative and effective hydrogels can be designed and implemented.

6. Conclusions

This review thoroughly examines how hydrogel surfaces contribute to the adsorption of organic and pharmaceutical pollutants, which are major hazards to human health and natural ecosystems. Since these contaminants are frequently persistent and biocompatible, creative and effective cleanup techniques are required. Natural and synthetic polymer-based hydrogels, metal–organic framework-based hydrogels, and nanomaterial-reinforced hydrogels have demonstrated exceptional efficacy in adsorbing organic and pharmaceutical contaminants. Their high porosity, tunable surface chemistry, different functional groups, different adsorption sites, and three-dimensional network architectures enable diverse interactions—π–π stacking, electrostatic attraction, stimuli-responsivity, and hydrophobic effects—that drive rapid and selective pollutant uptake. The control of its porous structure as well as its surface and functional groups significantly affect the hydrogel’s performance and efficiency in pollutant adsorption. Despite their advantages, challenges remain regarding the mechanical stability, regeneration potential, and cost-effectiveness of hydrogel-based adsorbents. To address these limitations, recent research has focused on developing hybrid hydrogels incorporating nanomaterials, responsive polymers, and bio-based components. Similarly, environmentally benign hydrogels derived from natural polymers such as alginate, cellulose, and chitosan offer sustainable alternatives for large-scale wastewater treatment applications. Although hydrogels are very good at removing contaminants from controlled synthetic solutions, they are difficult to use in actual wastewater because of the complicated matrix composition, which can compromise their adsorption efficacy and mechanical stability. To close the gap between lab-scale success and real-world applicability, future research must concentrate on enhancing hydrogel characteristics and carrying out more investigations in authentic environmental situations.

However, by offering fundamental insights into the design principles, adsorption processes, and practical issues of hydrogel-based adsorbents, this review paper is a useful tool for academics and practitioners. Future approaches for improving hydrogel performance are also highlighted, including the incorporation of multipurpose components, the creation of intelligent responsive systems, and the investigation of synergistic effects with various therapeutic modalities. Overall, the work highlights how hydrogel surfaces may be used as a flexible and successful tool for environmental cleanup.