Removal of Phenol derivatives from water systems with the use of silver nanoparticles

 Corresponding author: Varun Ravindra Mali Copyright © 2025 Author(s) retain the copyright of this article. This article is published under the terms of the Creative Commons Attribution Liscense 4.0. Removal of Phenol derivatives from water systems with the use of silver nanoparticles Varun Ravindra Mali * Kishinchand Chellaram College, Churchgate, Mumbai-400020, India. World Journal of Biology Pharmacy and Health Sciences, 2025, 21(03), 081-098 Publication history: Received on 26 January 2025; revised on 03 March 2025; accepted on 05 March 2025 Article DOI: https://doi.org/10.30574/wjbphs.2025.21.3.0241 Abstract Phenolic compounds and their derivatives represent a significant concern within the realm of water pollution, due to their pronounced toxicity, environmental persistence, and extensive utilization across industrial sectors. These pollutants emanate from a multitude of sources, including petrochemical industries, pharmaceutical production, and the manufacture of pesticides, thereby posing considerable threats to both aquatic ecosystems and human health. Conventional water treatment methodologies, encompassing biological degradation, adsorption, and chemical oxidation, frequently fall short in achieving complete elimination of these contaminants or may inadvertently generate secondary pollutants, thereby underscoring the need for the formulation of alternative treatments that are more effective and sustainable. Silver nanoparticles (AgNPs) have been recognized as an innovative material for the remediation of phenol and its derivatives, because of their high surface area, remarkable catalytic efficacy, and pronounced adsorption properties. The distinctive characteristics of AgNPs facilitate the efficient degradation and transformation of phenolic compounds through mechanisms such as adsorption, photocatalysis, and catalytic degradation redox reactions. Furthermore, the functionalization of AgNPs with stabilizing agents enhances their stability, selectivity, and reusability, thereby rendering them as a viable option for water purification. This article critically reviews recent progress in AgNP-based purification of waterbodies by removing phenols. Notable findings indicate that AgNPs substantially augment the rates of phenol degradation, with variables such as nanoparticle size, surface modifications, and prevailing environmental conditions playing pivotal roles in their operational efficiency. Nevertheless, challenges pertaining to nanoparticle aggregation, potential environmental toxicity, and the feasibility of large-scale application require further exploration. Future research should prioritize eco-friendly synthesis or green synthesis methodologies, enhanced recovery techniques, and the mitigation of potential risks linked to nanoparticle discharge. Keywords: Phenol; Toxicity to humans; Toxicity to environment; Silver nanoparticles; Green synthesis of silver nanoparticles 1. Introduction Water pollution in the environment has become a significant concern due to the toxic nature of phenolic derivatives which can adversely affect aquatic life and human health. Phenolic compounds are the organic compounds that are characterized by the hydroxyl group attached to them [1]. These compounds are widely recognized for their presence in industrial effluents and their potential to cause significant environmental harm due to their toxicity and persistence in water [2]. Phenolic derivatives are significant contributors to water pollution, as they are widely used in dye industries which discharge their organic dyes such as Azo dye (which contains nitrophenols). These dyes are highly stable, carcinogenic and non-biodegradable [5]. Furthermore, phenolic compounds can accumulate in food chain, contaminated water sources and food products; posing risk to human health [3]. Adsorption which is usually followed World Journal of Biology Pharmacy and Health Sciences, 2025, 21(03), 081-098 82 by oxidative degeneration has emerged as an effective technique to accelerate the treatment of dye effluent pollution. [6]. Dyes such as Rh B is also treated with this technique [4]. Phenols and the nitro derivatives of phenols were categorized as toxic pollutants by the US – EPA [7]. The nitrophenols besides odour problem causes colour change in many water bodies, which becomes the source of environmental damage of living organisms by decreasing sunlight, which disturbs the aquatic environment [8,9]. Phenolic contamination is majorly sourced by industrial effluents (by activities like petroleum refining, plastic manufacturing and effluents released from dye industries in the form of Azo-dyes which contain nitrophenols [9]. Phenol is used in chemical industries to produce derivatives such as alkylphenols, resins, aniline and cresols [11]. It also finds a noteworthy application in energy sector [10], pharmaceutical sector [35], textile, pesticides [54], and explosive industries from where the effluents or/and influents are discharged into the water bodies contributing in polluting them. As the phenolic compounds are highly reactive, they have the tendency of interacting and reacting with other components present in the aquatic environment such as the interaction of phenols with microbes [12,13] and with inorganic compounds [14,15]. Once in the environment, the phenolic compounds undergo metabolic transformation readily oxidising to quinone radicals that is known to have toxic effects on humans by forming covalent bonds with protein which can lead to DNA damage [16]. Other phenolic compounds such as chlorophenols, nitrophenols, aminophenol, bisphenol A are identified to have adverse effects on human [17]. The effects may include disrupting the development of mammary glands and delaying the onset of puberty in females [18,19], damage the vital organs like heart, kidney, liver, lungs and stomach [20-24]. Conventional methods like adsorption, chemical oxidation, microbial degradation, membrane separation [40,41,42] are used to reduce phenols from waste water, however due to the high polarity and chemical persistence of phenolic compounds these methods have relatively low efficiency (as low as 2040%) in reducing phenolic derivatives from water system [43]. Nanoparticles, especially purely made inorganic nanoparticles such as gold (Au), silver (Ag) and copper (Cu), have distinctive optical and photochemical properties which makes them ideal for reducing phenolic compounds in water [36]. Silver nanoparticles (AgNPs) due to its properties such as high surface area and reactivity [81], surface plasmon resonance (SPR) [82], surface-enhanced Raman scattering (SERS) [83,84] are used to remove phenol from water system by adsorption [82-90], catalytic degradation [91-101] and photocatalytic degradation [102-110] mechanism. The silver nanoparticles works as photocatalyst in degradation (oxidation) of phenolic derivation [37,38,39], when AgNPs is combined with metallic oxides such as TiO2 [106] and ZnO [107] enhances the photocatalytic degradation of phenol. The removal efficiency of phenol depends on size [92] and morphology [115-118] of AgNPs. Environmental factors such as pH [119,120], temperature [121-123] organic and inorganic contaminants [124-128] which blocks the active sites on the surface of AgNPs, reducing the removal efficiency of phenol. AgNPs released in the environment to remove phenol has toxic effects to mammals [129,130], non-mammals [131] and plants [132]. In this review the importance of green synthesis is highlighted as the green synthesis uses non-toxic and biodegradable reagents that can work as both reducing and capping agents which enhances the cost-effectiveness and reduce the toxicity of silver nanoparticles in the environment. More emphasis should be made to do relevant research on microbial mediated synthesis especially viral mediated synthesis of AgNPs [147]. The scope of this review is to discuss the effects and sources of phenol contamination. This review also discusses the use of silver nanoparticles to remove phenol and its derivatives from waste water, also highlighting the toxic effects of silver nanoparticles and the importance of using green synthesis which consist of plant and microbial mediated synthesis of silver nanoparticles. 2. Phenolic Derivatives: Sources, Properties and Environmental Impact 2.1. Sources of Phenolic Derivative in water The source of phenolic contamination in water can be categorized as Anthropogenic sources and Natural sources. 2.1.1. Anthropogenic Sources Due to their wide range of application, phenolic compounds are used in different industries such as chemical industries, coal and gas industries, dye, textile, explosive industries and pharmaceutical industries. It is used in chemical industries to produce other derivatives such as resins, aniline, cresols and alkylphenols [11]. Phenol serves as a vital part of many products like paints, medicine and lubricants. It is specially used as an intermediate product in the synthesis of phenolic compounds such as formaldehyde resins. It is also used to produce epoxide resins and polycarbonate which serves as a precursor for making plastic [47]. The pharmaceutical application of phenolic derivatives is remarkable as it used as a flexible precursor in drugs like aspirin and pharyngitis, cosmetics like sunscreen, skin lighting products and hair dye [48]. Phenolic derivatives find its application in wood distillation, water disinfection and for making paper, all this results in the formation of chlorophenols [46]. The phenolic contaminated effluents are discharged by the industries World Journal of Biology Pharmacy and Health Sciences, 2025, 21(03), 081-098 83 directly or indirectly which results in water pollution due to phenolic contamination. It is observed that the concentration of phenolic discharge from refineries is 6-500 mg L-1, cooking processes is 28-3900 mg L-1, coal and gas processing is 9-6800 mg L-1, pharmaceutical product is 0.1-1600 mg L-1 and petrochemical plants is 2.8-1220 mg L-1 [49-51]. In a study, second largest river of Ukraine, Dniester was studied, which reported that due to oil field nearby the river, phenolic concentration on the surface water of Dniester River was observed to be 0.009 and 0.081 mg L-1, whereas the reservoir contained an average of 0.026 mg L-1 of phenolic derivatives [52]. Phenols are the major constituents of pesticides, insecticides and herbicides which are the main agricultural sources of water pollution by phenolic compounds. Most pentachlorophenol [55] and chlorophenols such as 2,4-dichlorophenol, 2-chlorophenol are used in pesticides [53]. Other phenolic derivatives such as 2,4-dichlorophenoxyacetic acid are used as pesticides [54]. Pentachlorophenol degrades to other chlorophenols with lower chlorine substituents [55]. These pesticides, insecticides, herbicides and their degradation by products are often washed into the water system. 2.1.2. Natural Sources Plants, microorganism and decomposition of organic matter are recognized to be the natural sources responsible for contamination of phenolic derivatives in water bodies. Dead plants and animals in water bodies usually release phenolic compounds in water as the phenolic derivatives are components of many aquatic or terrestrial plant species. Few phenolic compounds are formed from amino acids present in hemicellulose of plants under the influence of ultraviolet light irradiation [56]. Phenylalanine which is a precursor for synthesis of phenols in plants is deaminated through phenylalanine ammonia-lysase catalysis and is converted to cinnamate. The obtained cinnamate is then converted to coumaric acid through hydroxylation catalyzed by cinnamate-4-hydroxylase. Coumaric acid is the starting material for producing flavonoids, stilbenes and furanocoumarins [63]. As these compounds are stored in roots and leaves, decomposition of dead roots and leaves introduces phenolic derivatives to soil and eventually into water bodies. The metabolic waste of human and other mammals also contains phenol [58] which is produced in the gut of human and other mammals as a result of tyrosine transformation [59]. It is well known fact that microorganism have the potential to degrade naturally occurring substrates into phenolic compounds [60]. Red and green marine algae contain macromolecules of phenolic derivatives [57]. In a study, Max et al. [61], reported that Debaryomyces hansenii metabolizes ferulic acid to produce 4-vinyl guaiacol, vanillic acid, vanillin and 4-ethylguaiacol. Another study by Shashwati et al. [62] reported Streptomyces sannanensis ability to convert ferulic acid to vanillic acid. All this contributes to contamination of phenol and its derivativities in water system. 2.2. Classification and Effects of Phenolic derivatives on Human 2.2.1. Classification of Phenolic Derivatives Based on the number of phenolic groups present in a molecule, phenolic compounds can be grouped as simple, bi and polyphenols. Phenolic derivatives such as hydroquinone, resorcinol, thymol, etc. have only one substituted phenolic ring and hence are simple phenols. These phenolic compounds are highly soluble in water due to the presence of single hydroxyl group, it often finds its application in disinfectants and chemical industries [64,66]. Biphenols are identified by the presence of two phenolic rings. These phenols have low solubility. Bisphenol A (BPA) is a common example of bi phenols, which is found in industrial waste and is known to have adverse effects on human health [66]. Compounds that consist of multiple units of phenolic structure are known as polyphenols; these are naturally found in plant (roots and leaves) [63]. Flavonoids, tannins, lignin and stilbenes are few examples of polyphenols [65]. The solubility of polyphenol is variable. Lower molecular weight polyphenols (e.g., flavonoids) are moderately soluble in water, on the other hand high molecular weight polyphenols (e.g., tannins) are poorly soluble in water and form colloidal suspension in water, however these are soluble in organic solvents such as ethanol [67]. 2.2.2. Toxicity to Humans Phenols are considered as one of the most toxic organic contaminants in waste water as they are highly toxic, even at low concentrations [69]. The European Union Directive 2455/2001/EC has set a limit of 0.5µgL-1 of phenolic concentration in drinking water with its individual concentration not exceeding 0.1µgL-1 [68]. Due to their nonbiodegradable nature, they remain in the aquatic system for a long time [70]. According American Conference of Government Industrial Hygienist (ACGIH) the exposure limit of phenol on skin should be less than 5 mgL-1 [72]. Exposing to high concentration of phenol to skin can cause blisters and burns [20]. Once in the system they metabolize and transform into reactive intermediate form such as quinone, which is known to form covalent bonds with protein, thus exhibiting toxic effects to humans [16]. For instance, catechols have the tendency to damage DNA by destroying some proteins in the body [16]. Phenols and its derivatives were identified as stable priority chemical pollutants by US Environmental Protecting Agency (EPA) [7], which are summarized in Table 1. Phenolic derivatives such as chlorocatechol, nitrophenol, chlorophenols and aminophenol are also categorized to exert toxic effects on humans [17]. Burning sensation in throat and mouth can be caused by chlorophenols [22]. Chlorophenols poisoning may also damage World Journal of Biology Pharmacy and Health Sciences, 2025, 21(03), 081-098 84 liver, kidney, lungs, digestive system and skin [23]. Bisphenol A which is a biphenol known to disrupt the development of mammary glands [18] and delay the onset of puberty in female [19]. Liquids that contain high phenolic concentration (1g) if ingested can cause fatality to humans as it may damage heart, kidney, liver and other internal vital organs [20]. In a study, Alshabib et al. [71] reported that acute phenol exposure may cause complication of central nervous system and exposure to fumes containing phenol can damage internal organs such as lungs, kidney, liver heart, and spleen. Due to these effects, strict limits on effluent discharge are amended. Therefore, an effluent discharge is limited to 0.05 mgL1 in the water system [72]; for non-chlorinated water it is 0.1 mgL-1 and chlorinated water is 0.001-0.002 mgL-1 [72,73]. Table 1 Eleven phenolic compounds considered priority pollutants by US-EPA Phenolic Structure Number of Phenolic unit Functional Group Additional groups Effect Ref Phenol (PH) 1 Hydroxyl Group - Skin irritation and burns, can damage kidney and liver, CNS depression. [22] 2-Chlorophenol (2-CP) 1 Hydroxyl Group Chlorine atom Skin and eye irritation, damage to vital organs like kidney and liver, when inhaled can cause respiratory problems. [23] 2,4-Dichlorophenol (2,4-CP) 1 Hydroxyl Group Chlorine atom Damage to liver and kidney, headache, nausea and long-term exposure can cause cancer. [23] 2,4,6-Trichlorophenol (2,4,6-TCP) 1 Hydroxyl Group Chlorine atom Endocrine disruption and headache. [18] Pentachlorophenol (PCP) 1 Hydroxyl Group Chlorine atom Damages liver, kidney and nervous system, endocrine disruption. [23] 2-Nitrophenol 1 Hydroxyl Group Nitro (-NO2) Group Reduces Oxygen supply in Blood, damages liver and kidney [3] World Journal of Biology Pharmacy and Health Sciences, 2025, 21(03), 081-098 85 (2-NP) and irritation of skin and eyes. 4-Nitrophenol (4-NP) 1 Hydroxyl Group Nitro (-NO2) Group Reduces Oxygen, damages liver. [5] 2,4-Dinitrophenol (2,4-DNP) 1 Hydroxyl Group Nitro (-NO2) Group Can cause hyperthermia, vomiting, nausea. [3] 4,6-Dinitro-o-cresol (4,6-DNOC) 1 Hydroxyl Group Nitro (-NO2) and methyl group Neurotoxic, skin irritation and metabolic disruption. [71] 2,4-Dimethylphenol (2,4-DMP) 1 Hydroxyl Group Methyl group Burning in respiratory tract and skin, damages liver. [20] 4-Chlorometacresol (4-CMC) 1 Hydroxyl group Chlorine atom and methyl group Damage to kidney and liver, skin burning. [20] 2.3. Environmental Impact Phenols cause damage to aquatic life when the concentration in the water system exceeds 1mgL-1 [71]. Due to the growing worldwide demand for textile products, these industries consume a high volume of water [76]. Phenols and their derivatives are used extensively in dyes such as Azo dyes that holds around 70% of market share [77]. Many of these dyes are known to have toxic effects on human health and environment, concerns regarding their toxicity and carcinogenicity have led to studies on toxicity of these dyes [74]. Due to dyes such as Dispersive blue 3 [77] which have low biodegradability, have toxicity and complex chemical structure, reduction of these effluents from environment is of great environmental concern [78]. Industrial activities have adversely affected the ecology of water system. Most of the water bodies near industrial zone has been contaminated by the effluents released by textile industries [79]. In a study Castro et al. [75], evaluated the toxicity of pre and post-treatment of textile effluents, on four different organisms, namely A.fischer, Raphidocelis subcapitata, Daphnia magna and Lemnar minor were used. Each of these organisms represented trophic levels. This study reviled that pre-treatment effluents have more toxic effects on organism as World Journal of Biology Pharmacy and Health Sciences, 2025, 21(03), 081-098 86 compared to when it was treated. However, treated effluents still exhibited toxicity towards D.magna. Due to high reactivity and non-biodegradability, phenolic compounds in water system have the tendency to react with other components of aquatic environment. Certain bacteria are known to degrade non-ionic surfactants into more toxic and harmful phenolic derivatives such as alkyl phenolic compounds. For example, bacteria can degrade nonylphenol polyethoxylated which may result in nonylphenol [12]. Tetrachlorocatechol which is formed by bacteria mediated degradation of pentachlorophenol and chlorocatechol is identified to be the result of microbial degradation [13]. Radiation in the form of UV radiation from sun may result in the reaction between phenol and nitrite ion which forms 2-nitophenol and 4-nitrophenol in water system [14], 2-nitrophenol can also be produced by the interaction between hydroxyl radical and phenol in water system [80]. The reduced products are more toxic and poses greater threat to the environment. 3. Properties of AgNPs used for the removal of Phenol from Water System. Silver nanoparticles (AgNPs) possess unique physicochemical properties that enhances its effectiveness in wastewater treatment, which also includes removal of phenol and its derivatives from untreated anthropogenic effluents [11]. Some of the key properties contributing to their effectiveness includes: 3.1. High Surface Area and reactivity Silver nanoparticles, because of their small size exhibits high surface area and high surface-to-volume ratio, which enhances their reactivity with organic pollutants such as phenol and its derivatives. Increased surface area provides more active sites for adsorption and catalytic degradation. For instance, AgNPs are used to reduce 4-nitrophenol to 4aminophenol [81]. This makes silver nanoparticles an efficient material for removal of contaminants such as phenol and its derivatives from waste water. Catalytic reduction of 4-nitrophenol to 4-aminophenol. 3.2. Surface Plasmon Resonance (SPR) Differing from their bulk metal counterpart silver nanoparticles show unique properties such as strong surface plasmon resonance (SPR), which arises from the collective oscillation of conduction electrons in response to incident light. This property enhances the ability of AgNPs to interact with organic molecules facilitating photolytic and adsorption-based removal of phenolic compounds [82]. SPR also contributes to improve electron transfer, which is essential for oxidative degradation mechanisms. 3.3. Surface-Enhanced Raman Scattering (SERS) AgNPs exhibits surface-enhanced Raman scattering which remarkably amplifies the Raman signals of adsorbed molecules. This property of silver nanoparticles facilitates the detection and monitoring of phenol and its derivatives in water system [83]. Additionally, the interaction between AgNPs and phenolic contamination through SERS can improve degradation efficiency [84]. Due to these properties AgNPs are widely studied for their role in adsorption, catalytic oxidation and photodegradation of toxic organic pollutants such as phenol and its derivatives. 4. Removal of phenol from water bodies by using silver nanoparticles Water pollution is caused by phenol and its derivatives, due to their high toxicity and persistence have become a significant environment concern [5]. Silver nanoparticles (AgNPs) due to their excellent adsorptive, catalytic and photocatalytic properties have gained attention [44,45]. World Journal of Biology Pharmacy and Health Sciences, 2025, 21(03), 081-098 87 4.1. Adsorption Mechanism of phenol removal using silver nanoparticles The adsorption of phenol in waste water by silver nanoparticles is facilitated via surface interaction [82]. The AgNPs provide a high surface area to volume ratio, which allows phenol molecules in waste water to be adsorbed on the surface of AgNPs [82]. The attraction of phenol molecules in aqueous environment also depends on the surface charge of AgNPs which is often modified by stabilizers or surfactants such as sodium dodecyl sulfate (SDS), triton X-100 (non-ionic), polyvinyl pyrrolidone (PVP) [85]. This is often governed by: • Vander Waals Force: These are weak intermolecular forces between phenol molecules and AgNPs which helps in the adsorption of phenol molecules onto AgNPs [86]. • π-π Interaction: Phenol and its derivatives consist of aromatic rings which can interact with the metallic surface of AgNPs via π-electron closed interaction [87]. • Hydrogen bonding: Phenolic hydroxyl (-OH) group may form hydrogen bonds with hydroxylated AgNPs and form weak coordinated complexes [87]. The oxygen atom in hydroxyl group (-OH) acts as a donor, binding to silver ions, which provides stability and retention of phenol on nanoparticles surface [87]. Surface-modifiers such as graphene, silica or activated carbon provides more functional groups for interaction thus enhancing phenol adsorption [88]. AgNPs supported on zeolites or biochar increases the surface area and improve adsorption efficiency [89,90]. 4.2. Catalytic Degradation for removal of phenol by using silver nanoparticles Silver nanoparticles are widely used as catalyst for the degradation of organic contaminants like phenol and its derivatives, this is due to their small size which translated in high surface area, strong redox properties and their ability to generate reactive oxygen species (ROS) [81]. AgNPs acts as electron mediator and donor which facilitates oxidation and reduction reactions, resulting in breaking down of phenols into smaller less harmful compounds [93]. AgNPs enables the transfer of electrons between oxidants such as hydrogen peroxide or dissolved oxygen and phenol molecules [94]. This promotes the generation of ROS such as hydroxyl radicals (-OH) and superoxide radicals (O2-) which breaks the aromatic ring of phenol by attacking it predominantly at ortho and para position due to their high electron density [99,100]. AgNPs can facilitate reduction of oxidants such as hydrogen peroxide (H2O2) or Ferric ion by acting as an electron donor [95]. In Fenton-like reaction Fe3+ is reduced to Fe2+ which is essential for the generation of hydroxyl radicals which accelerates phenol degradation [96]. Fenton reaction, results in catechol ring opening which converts catechol into muconioc acid that is further oxidized to maleic and fumaric acid [91]. As the Fe and H2O2 concentration is high, all the intermediates convert to formic acid which then oxidizes to non-toxic CO2 and H2O in water bodies [92]. Fig 1. Illustrates the mechanism of phenolic degradation by Fenton-like reaction by using AgNPs. AgNPs mimics enzyme like activity of peroxidase enzyme, promoting ROS production without requiring biological enzymes [97]. AgNPs due to their high surface area catalysis the conversion of H2O2 into -OH and O2 allowing efficient adsorption, accelerating phenol degradation. Due to their stability under reaction conditions AgNPs can be better alternative to biological peroxidase to remove phenol and its derivatives from the water system [98]. World Journal of Biology Pharmacy and Health Sciences, 2025, 21(03), 081-098 88 Figure 1 Mechanism of degradation of Phenol and its derivatives by Fenton-like reaction by using silver nanoparticles 4.3. Photocatalytic Degradation of phenol and its derivatives with the use of silver nanoparticles Photocatalytic degradation of phenolic compound takes place with the use of silver nanoparticles in combination with metal oxides catalyst like TiO2 and ZnO [111]. This has emerged as an effective method to degrade phenol from water bodies [101]. Semiconductors such as TiO2 and ZnO absorbs photons through UV or visible light, exciting electrons (e-) from valence bond to conduction band, this electron hole pairs (h+) initiates photocatalytic degradation. AgNPs acts as an electron trap and helps in preventing recombination, ensuring more reactive species are generated. These conduction band electrons react with dissolved oxygen (O2) reducing it to superoxide anions (O2•), while valence band holes react with water (H2O) or hydroxide ions (OH-) to produce hydroxyl radical (OH•). These reactive radicals attack and convert organic pollutants such as phenol into carbon dioxide and water [102,103]. Titanium dioxide (TiO2) due to its high stability, non-toxicity and strong oxidation potential is widely used as photocatalyst [105]. However, because of its wide bandgap ( 3.2 eV for anatase phase), that mainly restricts its photocatalytic activity to UV light (ʎ < 387nm) [106]. AgNPs can be used to significantly improve TiO2 – based photocatalysis [106]. Like TiO2, ZnO has a wide bandgap (  3.37 eV) but it suffers from high electron-hole recombination. AgNPs enhances charge separation and visible light adsorption when combined with ZnO [107]. Owing to SPR effect, AgNPs can absorb visible light (400-700nm) and transfer energy to semiconductor material, improving electron excitation and extending the photolytic activity beyond UV range. This enhances phenol degradation under sunlight [82]. In a study, Stanley et al. [108] employed Ag-ZnO nanoparticles in powder form to degrade textile dye such as Methylene lue, methyl orange and rhodamine B dyes. It reported the removal efficiency of 98.5%. In another experiment, Thi et al. [109] reported 85% removal efficiency when Ag-ZnO nanostructures in powdered form were used to remove Methylene blue from water. Wang L et al. [110] conducted a study in which AgNPs were used to reduce Methyl Orange and Coomassie brilliant blue from waste water by photocatalytic degradation. It reported the removal efficiency of 60-70%. By this we can state that the combination of AgNPs with photocatalytic metal oxides enhances the removal of phenol. The degree of effectiveness of photocatalytic degradation relies heavily on catalyst dose, exposure time, solution pH and light intensity [101]. 5. Factors affecting the efficiency of phenolic removal using silver nanoparticles The removal of phenol and its derivatives usually depends on the size, morphology of nanoparticles and environmental factors such as pH, temperature, ionic strength and presence of competing pollutants. The size of silver nanoparticles plays a crucial role in removal of phenol and its derivatives. Smaller nanoparticles have high surface area to volume ratio which translates in high surface energy, resulting in increased number of active sites available for adsorption and catalytic degradation [85,92]. This enhanced surface, improves the interaction between AgNPs and phenolic molecules, World Journal of Biology Pharmacy and Health Sciences, 2025, 21(03), 081-098 89 which leads to higher removal efficiency [111]. However, agglomeration may occur in extremely small nanoparticles, reducing their effectiveness [112]. Therefore, it is necessary to use surfactants or capping agents like Tween 20 and Polyvinyl pyrrolidone to maintain optimal size for maximum efficiency [113,114]. AgNPs of varying morphologies such as spherical, triangular, cubic and rod-like nanoparticles are used due to the difference in surface area and active sites available which affect the catalytic and adsorption ability [115]. Among these spherical AgNPs are used widely due to their uniform distribution and stability. These nanoparticles provide moderate reactivity and are effective in adsorption-based removal [116]. Rod-shaped AgNPs have enhanced surface area as compared to spherical AgNPs, which enhances its interaction with organic pollutants like phenolic compounds. However, the efficiency of nanostructures depends on dispersion stability [118]. Triangular and cubic AgNPs have high surface energy as they have more exposed facets, therefore they tend to exhibit better catalytic and adsorptive properties as compared to spherical silver nanoparticles [117]. Optimization of both morphology and size of AgNPs is essential for high efficiency in phenolic removal. Environmental factors such as pH and temperature greatly affect the efficiency of phenolic removal from the water system with the use of AgNPs. The pH of the water system affects the surface properties of silver nanoparticles and the chemical state of phenolic compounds [119]. Typically, slightly acidic to neutral conditions (pH 5-7) increases the efficiency of phenol removal [121]. At high pH (basic) phenols tend to ionize into phenolate anions, that has negative charges, as AgNPs often develop negative surface charge, which may cause electrostatic repulsion that reduces adsorption efficiency [120]. Like pH, adsorption, catalytic degradation and photocatalytic degradation of phenolic compounds is highly dependent on temperature. At high temperature the kinetic energy of molecules increases which enhances their interaction with AgNPs [121]. High temperatures with light exposure increase the removal efficiency of photocatalytic degradation of phenol in the water system [122]. However, excessive temperature may destabilize AgNPs which may lead to agglomeration and reduced surface area, thereby lowering efficiency of removal [123]. Other than pH and temperature the contaminants present in waste water alters the removal efficiency of phenol. Organic pollutants such as humic acid and fulvic acid are dissolved in water that can compete with phenol for adsorption sites on AgNPs [124]. These relatively large organic compounds from coating on the surface of AgNPs reducing phenolic removal, by blocking the active sites [125] disrupting the catalytic degradation of phenol and its derivatives in water. Some of the organic compounds like dyes, pesticides and pharmaceuticals reacts with phenolic compounds or alters the surface charge of AgNPs which may lead to unpredictable interaction that affects removal efficiency [126], heavy metals such as Pb2+, Ca2+ and Cd2+ may catalytically reduce AgNPs, reducing their effectiveness [127]. Chlorides, sulfates and phosphates present in water bodies may react with AgNPs to form insoluble compounds like AgCl and AgSO4, that decreases the available sites and surface area for phenol adsorptions [128]. 6. Challenges and Future Perspectives Environmental conditions such as high pH and high temperature may destabilize AgNPs and ionize phenol into phenolate anions which may decrease the removal efficiency of phenol in waste water [120,121]. AgNPs used in the removal of phenol and its derivatives when released into the environment have toxic effects on mammals, nonmammals and plants. The toxic effects on mammals were studied in an experiment in which Sprague-Dawley rats were exposed to AgNPs of size 18 nm of dose concentration 2.9X106 particles cm-3. This experiment resulted in lung inflammation and damage to respiratory system [129]. In another similar study conducted by Kim et al. [130], SpragueDawley rats were ingested with AgNPs of size 60 nm which are mixed with diet for 28 weeks, which resulted in accumulation of AgNPs in kidney. In an experiment demonstrating the toxic effects of AgNPs on non-mammals, Zebrafish embryos were exposed to AgNPs of size 11.6 nm ± 3.5 nm for 120 hours. This resulted in 100 percent mortality and malformation of embryonic morphology [131]. A study on Lolium Multiflorum was conducted to evaluate the effects of AgNPs on plants in which L. multiflorum was exposed to AgNPs of size 6nm with concentration 14 mgL-1 for 7-21 days. This study reported the decrease in root and shoot length [132]. Often the conventional methods used to synthesize AgNPs can be costly [133] and may require the use of harmful chemical such as NaBH4 [134], dimethyl formamide (DMF) [135] and hydrazine [136] which acts as strong reducing agents, that reduces silver ions to metallic silver. Hydrazine has been ranked as a potential carcinogen by United Staes Environmental Protection Agency [137]. Other than that, AgNPs reacts with phenol and form other phenolic derivatives such as quinone which is known to form covalent bonds with proteins, thus exhibiting adverse effects to humans [16]. Silver nanoparticles need to be modified by metallic oxides like TiO2 [104] and ZnO [108,109] to enhance its catalytic and photocatalytic degradation for the World Journal of Biology Pharmacy and Health Sciences, 2025, 21(03), 081-098 96 [96] Friedrich, L. C., Zanta, C. L. de P. e S., Machulek, A., Silva, V. O., & Quina, F. H. (2012). Interference of inorganic ions on phenol degradation by the Fenton reaction. Scientia Agricola, 69(6), 347–351. https://doi.org/10.1590/S0103-90162012000600001 [97] Miranda, R. R., Oliveira, A. C. S., Skytte, L., Rasmussen, K. L., & Kjeldsen, F. (2022). Proteome-wide analysis reveals molecular pathways affected by AgNP in a ROS-dependent manner. Nanotoxicology, 16(1), 73–87. https://doi.org/10.1080/17435390.2022.2036844 [98] Rani, P., Kumar, V., Singh, P., Matharu, A. S., Zhang, W., Kim, K.-H., Singh, J., & Rawat, M. (2020). Highly stable AgNPs prepared via a novel green approach for catalytic and photocatalytic removal of biological and nonbiological pollutants. Environment International, 143, 105924. https://doi.org/10.1016/J.ENVINT.2020.105924 [99] Ahmed, O. H., Altarawneh, M., Al-Harahsheh, M., Jiang, Z.-T., & Dlugogorski, B. Z. (2020). Formation of phenoxytype Environmental Persistent Free Radicals (EPFRs) from dissociative adsorption of phenol on Cu/Fe and their partial oxides. Chemosphere, 240, 124921. https://doi.org/10.1016/J.CHEMOSPHERE.2019.124921 [100] Terzian, R., Serpone, N., & Fox, M. A. (1995). Primary radicals in the photo-oxidation of aromatics — reactions of xylenols with •OH, N3• and H• radicals and formation and characterization of dimethylphenoxyl, dihydroxydimethylcyclohexadienyl and hydroxydimethylcyclohexadienyl radicals by pulse radiolysis. Journal of Photochemistry and Photobiology A-Chemistry, 90, 125–135. https://doi.org/10.1016/1010-6030(95)04090-3 [101] Laoufi N., Tassalit D. and Bentahar F. The degradation of phenol in water solution by TiO2 photocatalysis in a helical reactor. Global NEST Journal 10, pp. 404–418, 2008 [102] Dursun G., Cicek H. and Dursun A. Y. Adsorption of phenol from aqueous solution by using carbonised beet pulp. Journal of Hazardous Materials 125, pp. 175–182, 2005 [103] Ugurlu M., Gurses A., Yalcin M. and Dogar C. Removal of phenolic and lignin compounds from bleached kraft mill effluent by fly ash and sepiolite. Adsorption 11, pp. 87–97, 2005 [104] Shahrezaei F., Akhbari A. and Rostami A. Photodegradation and removal of phenol and phenolic derivatives from petroleum refinery wastewater using nanoparticles of TiO2.International Journal of Energy and Environment 3, pp. 267–274, 2012. [105] Pitre, S. P., Yoon, T. P., & Scaiano, J. C. (2017). Titanium dioxide visible light photocatalysis: surface association enables photocatalysis with visible light irradiation. Chemical Communications, 53(31), 4335–4338. https://doi.org/10.1039/C7CC01952 [106] V. Vaiano, O. Sacco, M. Stoller, A. Chianese, P. Ciambelli and D. Sannino, Int. J. Chem. React.Eng. 12, 63 (2014). doi:10.1515/ijcre-2013-0090. [107] Cassaignon, S., Colbeau-Justin, C., & Durupthy, O. (2013). Titanium Dioxide in Photocatalysis (pp. 153–188). Springer, London. https://doi.org/10.1007/978-1-4471-4213-3_6 [108] Stanley, R.; Jebasingh, J.A.; Manisha Vidyavathy, S.; Stanley, P.K.; Ponmani, P.; Shekinah, M.; Vasanthi, J. Excellent Photocatalytic degradation of Methylene Blue, Rhodamine B and Methyl Orange dyes by Ag-ZnO nanocomposite under natural sunlight irradiation. Optik 2021, 231, 166518. [CrossRef] [109] Thi, V.H.T.; Cao, T.H.; Pham, T.N.; Pham, T.T.; Le, M.C. Synergistic Adsorption and Photocatalytic Activity under Visible Irradiation Using Ag-ZnO/GO Nanoparticles Derived at Low Temperature. J. Chem. 2019, 2019, 2979517. [CrossRef] [110] Wang, L.; Lu, F.; Liu, Y.; Wu, Y.; Wu, Z. Photocatalytic degradation of organic dyes and antimicrobial activity of silver nanoparticles fast synthesized by flavonoids fraction of Psidium guajava L. leaves. J. Mol. Liq. 2018, 263, 187–192. [CrossRef] [111] Denrah, S., & Sarkar, M. (2019). Design of experiment for optimization of nitrophenol reduction by green synthesized silver nanocatalyst. Chemical Engineering Research & Design, 144, 494–504. https://doi.org/10.1016/J.CHERD.2019.02.021 [112] Naidu, L. D., Saravanan, S., Goel, M., Periasamy, S., & Stroeve, P. (2016). A novel technique for detoxification of phenol from wastewater: Nanoparticle Assisted Nano Filtration (NANF). Iranian Journal of Environmental Health Science & Engineering, 14(1), 9. https://doi.org/10.1186/S40201-016-0249-8 [113] Orrico, M., Pota, G., Venezia, V., de Gennaro, B., Landi, G., Tescione, F., Pezzella, A., Luciani, G., & Silvestri, B. (2024). Phenolic driven decoration of silica with Ag nanoparticles: Towards sustainable water remediation. Journal of Water Process Engineering. https://doi.org/10.1016/j.jwpe.2024.105079 World Journal of Biology Pharmacy and Health Sciences, 2025, 21(03), 081-098 97 [114] Gasser, C. A., Yu, L., Svojitka, J., Wintgens, T., Ammann, E. M., Shahgaldian, P., Corvini, P. F.-X., Corvini, P. F.-X., & Hommes, G. (2014). Advanced enzymatic elimination of phenolic contaminants in wastewater: a nano approach at field scale. Applied Microbiology and Biotechnology, 98(7), 3305–3316. https://doi.org/10.1007/S00253-0135414-8 [115] Nguyen, T.-D., Nguyen, T.-D., Dang, C.-H., & Mai, D.-T. (2018). Biosynthesized AgNP capped on novel nanocomposite 2-hydroxypropyl-β-cyclodextrin/alginate as a catalyst for degradation of pollutants. Carbohydrate Polymers, 197, 29–37. https://doi.org/10.1016/J.CARBPOL.2018.05.077 [116] Tang, H., Meng, G., Huang, Q., Zhang, Z., Huang, Z., & Zhu, C. (2012). Arrays of Cone-Shaped ZnO Nanorods Decorated with Ag Nanoparticles as 3D Surface-Enhanced Raman Scattering Substrates for Rapid Detection of Trace Polychlorinated Biphenyls. Advanced Functional Materials, 22(1), 218–224. https://doi.org/10.1002/ADFM.201102274 [117] Ajitha, B., Kumar Reddy, Y. A., Kim, M. J., Jeon, H.-J., & Ahn, C. W. (2016). Superior catalytic activity of synthesized triangular silver nanoplates with optimized sizes and shapes. Catalysis Science & Technology, 6(23), 8289–8299. https://doi.org/10.1039/C6CY01948J [118] Fernando, I., & Zhou, Y. (2019). Impact of pH on the stability, dissolution and aggregation kinetics of silver nanoparticles. Chemosphere, 216, 297–305. https://doi.org/10.1016/J.CHEMOSPHERE.2018.10.122 [119] Gebru, H., Cui, S., Li, Z., Wang, X., Xianfu, P., Liu, J., & Guo, K. (2017). Facile pH-Dependent Synthesis and Characterization of Catechol Stabilized Silver Nanoparticles for Catalytic Reduction of 4-Nitrophenol. Catalysis Letters, 147(8), 2134–2143. https://doi.org/10.1007/S10562-017-2100-Y [120] Caetano, M., Valderrama, C., Farran, A., & Cortina, J. L. (2009). Phenol removal from aqueous solution by adsorption and ion exchange mechanisms onto polymeric resins. Journal of Colloid and Interface Science, 338(2), 402–409. https://doi.org/10.1016/J.JCIS.2009.06.062 [121] Vasudevan, S. (2014). An efficient removal of phenol from water by peroxi-electrocoagulation processes. Journal of Water Process Engineering, 2, 53–57. https://doi.org/10.1016/J.JWPE.2014.05.002 [122] Mohan, G. K., Meenachi, S., Kiruthika, K., & Kirthiga, D. (2025). Unveiling Optimal Conditions for Phenol Degradation: Response Surface Methodology and ANOVA Analysis of ZnO and Ag-Doped ZnO Photocatalysts. Nature Environment and Pollution Technology, 24(S1), 293–303. https://doi.org/10.46488/nept.2024.v24is1.022 [123] Su, Y., Zhao, X., Bi, Y., & Han, X. (2019). ZnO/Ag–Ag2O microstructures for high-performance photocatalytic degradation of organic pollutants. Clean Technologies and Environmental Policy, 21(2), 367–378. https://doi.org/10.1007/S10098-018-1641-0 [124] Bhatia, M., & Charola, S. (2019). Phenol removal using non-ionic resin bed from industrial waste water. International Journal of Advance Research and Innovative Ideas in Education, 5(3), 601–606. https://www.ijariie.com/FormDetails.aspx?MenuScriptId=149174 [125] Anku, W. W., Mamo, M. A., & Govender, P. P. (2017). Phenolic Compounds in Water: Sources, Reactivity, Toxicity and Treatment Methods. IntechOpen. https://doi.org/10.5772/66927 [126] kumar, L. (2021). The Removal of phenolic compounds from water using natural adsorbnt. 1(01). [127] Hamad, H. T. (2021). Removal of phenol and inorganic metals from wastewater using activated ceramic. Journal of King Saud University: Engineering Sciences, 33(4), 221–226. https://doi.org/10.1016/J.JKSUES.2020.04.006 [128] Shi, H.-X., Liu, S., Guo, J., Fang, F., Chen, Y., & Yan, P. (2023). Potential Role of Agnps within Wastewater in Deteriorating Sludge Floc Structure and Settleability During Activated Sludge Process: Filamentous Bacteria and Quorum Sensing. https://doi.org/10.2139/ssrn.4577974 [129] J. H. Sung, J. H. Ji, J. U. Yoon, D. S. Kim, M. Y. Song, J. Jeong,B. S. Han, J. H. Han, Y. H. Chung, J. Kim, T. S. Kim,H. K. Chang, E. J. Lee, J. H. Lee and I. J. Yu, Inhalation Toxicol., 2008, 20, 567–574. [130] Y. S. Kim, J. S. Kim, H. S. Cho, D. S. Rha, J. M. Kim, J. D. Park, B. S. Choi, R. Lim, H. K. Chang, Y. H. Chung, I. H. Kwon, J. Jeong, B. S. Han and I. J. Yu, Inhalation Toxicol., 2008, 20, 575–583. [131] O. Bar-Ilan, R. M. Albrecht, V. E. Fako and D. Y. Furgeson,Small, 2009, 5, 1897–1910. [132] dx.doi.org/10.1021/es103995x World Journal of Biology Pharmacy and Health Sciences, 2025, 21(03), 081-098 98 [133] Zhang, K.-X.; Wen, X.; Yao, C.-B.; Li, J.; Zhang, M.; Li, Q.-H.;Sun, W.-J.; Wu, J.-D. Chem. Phys.Lett.2018,698,147–151. doi:10.1016/j.cplett.2018.03.018 [134] Solanki, J. N.; Murthy, Z. V. P. Ind. Eng. Chem. Res. 2011, 50,7338–7344. doi:10.1021/ie200536q [135] Iravani, S.; Korbekandi, H.; Mirmohammadi, S. V.; Zolfaghari, B. Res. Pharm. Sci. 2014, 9, 385–406 [136] Goswami, S.; Aich, K.; Das, S.; Basu Roy, S.; Pakhira, B.; Sarkar, S. RSC Adv. 2014, 4, 14210–14214. doi:10.1039/c3ra46663a [137] EPA, Risk Information System Division. Chemical AssessmentSummary of Hydrazine/Hydrazine sulfate. https://cfpub.epa.gov/ncea/iris2/chemicalLanding.cfm?substance_nmbr=352. [138] Sharma, V. K.; Yngard, R. A.; Lin, Y. Adv. Colloid Interface Sci. 2009,145, 83–96. doi:10.1016/j.cis.2008.09.002 [139] Rafique, M.; Sadaf, I.; Rafique, M. S.; Tahir, M. B. Artif. Cells, Nanomed., Biotechnol. 2017, 45,12721291.doi:10.1080/21691401.2016.1241792 [140] Srikar, S. K.; Giri, D. D.; Pal, D. B.; Mishra, P. K.; Upadhyay, S. N. Green Sustainable Chem.2016,6,34–56. doi:10.4236/gsc.2016.61004 [141] Iravani, S. Green Chem. 2011, 13, 2638–2650. doi:10.1039/c1gc15386b [142] Khan, M.; Shaik, M. R.; Adil, S. F.; Khan, S. T.; Al-Warthan, A.;Siddiqui, M. R. H.; Tahir, M. N.; Tremel, W. Dalton Trans. 2018, 47,11988–12010. doi:10.1039/c8dt01152d [143] Gardea-Torresdey, J. L.; Gomez, E.; Peralta-Videa, J. R.; Parsons, J. G.; Troiani, H.; Jose-Yacaman, M. Langmuir 2003, 19, 1357–1361. doi:10.1021/la020835i [144] Lee, S.-Y.; Royston, E.; Culver, J. N.; Harris, M. T. Nanotechnology 2005, 16, S435–S441. doi:10.1088/09574484/16/7/019 [145] Thangavelu, R. M.; Ganapathy, R.; Ramasamy, P.; Krishnan, K.Arabian J. Chem. 2020, 13, 2750–2765. doi:10.1016/j.arabjc.2018.07.006 [146] Sastry, M.; Ahmad, A.; Islam Khan, M.; Kumar, R. Curr. Sci. 2003, 85,162–170 [147] Shahverdi AR, Minaeian S, Shahverdi HR, Jamalifar H, Nohi A. Rapid synthesis of silver nanoparticles using culture supernatants of Enterobacteria: A novel biological approach Process Biochemistry. 2007;42:919-923. [148] Zhang, Q., Li, J. X., & Xu, M. (2022). Ag-decorated ZnO-based nanocomposites for visible light-driven photocatalytic degradation: basic understanding and outlook. 55(48), 483001. https://doi.org/10.1088/1361-6463/ac941a