1. Introduction
One of the major classes of emerging contaminants that is particularly gaining attention in recent times is pharmaceuticals and personal care products owing to their potential threats to humans and the ecosystem. Antibiotics are either natural or synthetic compounds that have the ability to inhibit the growth and metabolic functions of microorganisms. These are extensively utilized in the fields of medicine, aquaculture, agriculture, and animal husbandry and are known to pose significant threats to the environment and also to human health [1]. Based on their mode of action and chemical structure they are classified into several classes such as β-lactams, tetracyclines, sulfonamides, fluoroquinolones, macrolides, and others [2].
Antibiotic consumption is increasing globally due to two factors: (i) an Increase in population, and (ii) an increase in demand for animal protein [3]. Significant levels of antibiotics enter the environment and pose a risk to aquatic organisms like freshwater algae, zooplankton, fishes, and micro or macrophytes [4]. Antibiotics released into the environment may be present in concentrations ranging from ng/L to μg/L. Antibiotics can induce chronic effects in aquatic organisms such as alteration in growth, reproduction, and behaviour [5].
The widespread use of antibiotics has caused a surge in antibiotic-resistant bacteria and antibiotic-resistant genes in the environment [6]. These can pose a serious threat not only to the environment but also to human health. Excessive use of antibiotics may lead to allergic reactions, poisoning, coma, and shock, and in severe cases, it may result in death [7]. Excessive use of antibiotics is also associated with the disruption of microbial balance in the gut [1].
A significant portion of antibiotics administered in animals are released into the environment as the animal intestine has a limited capacity to absorb the antibiotics [8]. In recent times, concern regarding the presence of antibiotics in the environment due to their extensive use in animal husbandry has gained attention [1]. Several efforts have been made to explore the alternatives to veterinary antibiotics which include probiotics, enzymes, and plant extracts but the extent to which these substitutes could replace antibiotics remains inadequately understood [1, 9–11]. Since antibiotics have numerous benefits, their complete prohibition is not possible. Therefore, it is necessary to devise efficient methods for addressing existing antibiotic pollution.
Traditional antibiotic remediation methods like membrane filtration, advanced oxidation, coagulation, and chlorination have been widely used to treat antibiotic contamination in wastewater [12, 13]. Adding chemicals (like iron or aluminium salts) to destabilize microorganisms and suspended particles and produce easily removed flocs is known as coagulation [14]. Although coagulation can be effective in getting rid of some antibiotics and organic pollutants, it is usually limited by its inability to specifically target certain antibiotic compounds and the production of sludge that must be disposed of [15]. Advanced oxidation processes (AOPs) like ozonation, Fenton’s reagent, and UV/H2O2 systems use highly reactive species to convert antibiotics into less harmful forms. However, because AOPs can be costly, energy-intensive, and require precise control over operating parameters, they are not practical for large-scale applications [16, 17]. Although the traditional chlorination method, which uses chlorine or chlorine dioxide, is effective against a number of infections, it frequently results in dangerous byproducts like trihalomethanes that are far more harmful to the environment [18]. Finally, by letting antibiotics pass through semi-permeable membranes, membrane filtration physically extracts them from water. Reverse osmosis and nanofiltration are two steps in this process. Although membrane filtration is effective at removing a wide range of contaminants, it is energy-intensive, prone to fouling, and can be costly to operate and maintain [19]. The challenges that each of these conventional methods faces in terms of cost, efficiency, by-product production, and scalability underscore the need for more targeted and sustainable approaches in antibiotic remediation.
Nature-based solutions offer promising avenues for the removal of antibiotics from the environment, leveraging natural processes and ecological interactions to address emerging contaminants effectively. For instance, wetlands and constructed wetlands can act as natural filters, where microbial communities degrade antibiotics through biodegradation processes. Nature-based solutions not only offer sustainable approaches to antibiotic removal but also promote biodiversity conservation and ecosystem resilience, underscoring their importance in safeguarding environmental and human health.
This review provides a piece of comprehensive knowledge about antibiotics and the pollution caused by them. It explores the various sources via which these antibiotics enter the ecosystem. Moreover, the review deals with the environmental toxicity associated with antibiotics focussing on their harmful effects on the environment and human health. Additionally, the review delves into nature-based strategies such as constructed wetlands, phytoremediation, and microbial degradation utilized for mitigating antibiotic pollution. All these natural methods support the biological environment and aid in effectively degrading antibiotics from the environment. Finally, the review addresses the challenges, opportunities, and future perspectives associated with the implementation of these nature-based solutions for effectively combating antibiotic pollution. The review concludes with insights into the potential benefits of these nature-based strategies in enhancing environmental sustainability.
2. Methodology
A comprehensive literature search was carried out using databases such as Scopus, science direct, web of science, and google scholar and the trend is given in supplementary materials in Fig. S1. The major keywords used for searching the articles were antibiotic pollution, wastewater remediation, nature-based solution, phytoremediation, microbial degradation, constructed wetlands, plant-based treatments. The review articles, research articles and case studies focussing on nature-based approaches for remediation of antibiotic contaminated wastewater are some of the inclusion criteria that were selected for this review. On the other hand, articles not related to antibiotic removal and those not focused on nature-based solutions were not included in this review. Initially a total of more than 150 articles were identified and after removing the duplicate articles and applying the inclusion and exclusion criteria, 88 articles were evaluated and are included in this review based on their relevance to the topic.
3. Sources of Antibiotic Pollution
There are several sources of antibiotic pollution in aquatic environments which are mainly divided into five major groups namely discharge from hospitals and pharmaceuticals, wastewater discharge from livestock farms, municipal waste, discharge from wastewater treatment plants, and landfill leachates. All these sources are depicted in supplementary materials in Fig. S2. The occurrence and concentration ranges of common antibiotics detected in various wastewater sources across different geographical regions is given in Table S1 of supplementary materials.
3.1. Effluent Discharge from Hospitals and Pharmaceutical Sectors
One of the major sources of antibiotic pollution is hospital wastewater due to its complex nature as it contains several toxic components such as pathogens, heavy metals, pharmaceutical residues, coliforms, and radioelements [20, 21]. Numerous studies have shown the presence of several antibiotics such as norfloxacin, sulfamethoxazole, cefazolin, cephalexin, ofloxacin, roxithromycin, lomefloxacin, and trimethoprim in considerable amounts (μg/L) in hospital wastewater [22, 23]. The concentration of major antibiotics in hospital wastewater such as ofloxacin, ciprofloxacin, and trimethoprim was found to be in the range of 66.9 to 1262 ng/l in Malaysia which was quite lower than that found in Vietnam and China [24]. The amount of antibiotics in hospital wastewater was comparable to that found in countries such as India and other European nations [21]. High concentration of antibiotics such as sulfamethoxazole and trimethoprim (up to 100–250 μg/L) is found in wastewater samples discharged from pharmaceutical manufacturing units in Vietnam [25]. Usually, the antibiotics are found in the range of 100–1000 ng/l in effluents discharged from pharmaceutical units. The presence of antibiotics in wastewater discharged from hospitals and pharmaceutical sectors is mainly due to their high production rate and persistence in the environment [26].
3.2. Waste Discharge from Livestock Farms
The presence of antibiotics in wastewater discharged from livestock farms has been extensively found in countries like Vietnam, China, and Thailand. The antibiotic concentration ranged from a few ng/L to 170 μg/L in wastewater discharged from pig and poultry farms [21, 27, 28]. The predominant antibiotics found in livestock wastewater were lincomycin and oxytetracycline [27, 28]. Relatively lower concentration (109 ng/L) of oxytetracycline, ciprofloxacin, enrofloxacin, erythromycin, and ofloxacin was found in wastewater from a zoological park in Perak, Malaysia [24].
3.3. Municipal Waste
The discharges from household activities, commercial sectors as well as surface runoffs such as stormwater are collected by sewer systems that are transferred to wastewater treatment plants. Due to excess stormwater and sewage overflow, pollution occurs [29]. A wide concentration of several antibiotics is found in sewer water in concentrations ranging from ng/L to nearly 50 μg/L [21]. The antibiotics (Sulfamethoxazole, Lincomycin, Erythromycin, Trimethoprim, Ciprofloxacin, Enrofloxacin, Norfloxacin, Ofloxacin) are usually detected in high concentrations in sewage waters of Vietnam, Indonesia, Malaysia, and Philippines [21, 30, 31]. B-lactams on account of their low stability and undetectable concentrations are not found in surface waters. Cephalexin and Cefazolin were found in drainage waters of urban areas in the concentration range of μg/L [27, 30].
3.4. Discharge from Wastewater Treatment Plants
Several studies have investigated the removal efficiency of various types of wastewater treatment plants for antibiotics in various countries like China, Vietnam, Thailand, Singapore, and South Korea. The concentration of both influent and effluent water has been found to have antibiotics in concentrations ranging from a few ng/L to 44 mg/L. In one of the study by [32], it was reported that the concentration of Lincomycin in the effluent water of wastewater treatment plants was in the range of 2.43 μg/L to 44 mg/L. Also, the concentration of Sulfathiazole, Sulfamethazine, and Chlortetracycline was 159 μg/L, 115 μg/L, and 33.5 μg/L, respectively in the effluents from wastewater treatment plants [33]. The concentration of antibiotics ranged from 400 to 21500 ng/L in the effluents discharged from wastewater treatment plants in Vietnam [34] which was quite high as compared to effluents discharged from wastewater treatment plants in Thailand (1–1500 ng/L) [35]. In China, the concentration was found in the range of 1 to 2000 ng/L. The concentrations of antibiotics differ in different locations on account of variations in input flow and treatment methodologies. Advanced treatment methodologies such as membrane bioreactors demonstrate better removal efficiency for antibiotics as compared to conventional activated sludge processes [30, 33].
3.5. Landfill Leachates
Currently, there is limited information on the release and environmental loads of antibiotics from landfill sites. It has been found that the concentration of antibiotics in leachates from various municipal solid waste landfill sites in China ranges from a few ng/L to 40 μg/L [28, 36, 37]. It was observed that the concentration of antibiotics was higher in fresh leachates as compared to middle-aged and old leachates [28, 36, 37]. Several factors such as disposal activities, landfill matrix, type of waste, and fate of specific compounds affect the levels of antibiotics in leachates. There is a need to develop appropriate treatment methods for reducing the amount of antibiotics in landfill leachates like aerobic biological treatment methods and constructed wetlands [21, 36].
4. Environmental Toxicity of Antibiotics
Antibiotics are considered persistent in the environment because they enter the environment at a rate that exceeds their rate of elimination. This pollution poses significant threats to microorganisms, animals, plants, and other living organisms. To assess the hazard of antibiotics in the environment, the predicted environmental concentration (PEC), or measured environmental concentration (MEC), is compared to the predicted no-effect concentration (PNEC) [38]. Toxicity can be indicated by LC50, EC50, and IC50 values. Antibiotics negatively impact non-target organisms such as algae, zebrafish, mussels, Daphnia, and other aquatic species [39–41]. Their toxic effects begin at the molecular level and can escalate to cellular, organismal, population, community, and ecosystem levels.
Blue-green algae (cyanobacteria) are prokaryotes and thus sensitive to antibiotics due to their similarity to pathogenic bacteria [42]. Antibiotics’ known modes of action on bacteria explain their effects on cyanobacteria. Although green algae are eukaryotes, antibiotics can still harm them because of the prokaryotic origins of their chloroplasts and mitochondria [42]. This toxicity in green algae is linked to the inhibition of chloroplast functions such as protein synthesis and photosynthesis, which affects cell growth [43]. For example, the cyanobacterium Microcystis aeruginosa is significantly more sensitive to fluoroquinolones than the green alga Pseudokirchneriella subcapitata [40]. The EC50 values for ciprofloxacin were 0.005 mg/L for M. aeruginosa and 1.1 mg/L for P. subcapitata. M. aeruginosa is also more sensitive to erythromycin and oxytetracycline, but not to tetracycline. Prolonged exposure increases toxicity, with lower EC50 values observed over longer periods.
Many antibiotics inhibit photosynthesis by blocking the photosystem II electron transport chain [42, 44]. Additionally, excited chlorophyll molecules can generate reactive oxygen species (ROS), causing oxidative stress. ROS removal is managed by enzymatic antioxidants like catalase, superoxide dismutase, and glutathione (GSH)-specific peroxidase, as well as enzymes in the ascorbate-GSH cycle and non-enzymatic antioxidants such as ascorbate and GSH. One study found erythromycin to be the most toxic to the antioxidant system of P. subcapitata, significantly decreasing ascorbic acid (ASA) and GSH levels. ASA and GSH help eliminate ROS and regulate redox homeostasis. [44] suggested that erythromycin disrupts ASA and GSH biosynthesis, causing oxidative stress.
In short-term toxicity tests, antibiotics showed low acute toxicity to the luminescent marine bacterium Vibrio fischeri [45]. The EC50 values exceeded 20 mg/L for sulfamethoxazole and oxytetracycline, and over 100 mg/L for erythromycin, ofloxacin, ciprofloxacin, and amoxicillin in assays lasting 15 to 30 minutes [45, 46]. This insensitivity is likely due to the short exposure time. However, in a 24-hour assay, V. fischeri showed toxicity at environmentally relevant concentrations of 81 μg/L oxytetracycline and 0.014 μg/L ofloxacin [42, 47].
Genotoxicity studies have shown that antibiotics can damage DNA, serving as a sensitive biomarker for toxicity. Macrolides, fluoroquinolones, and tetracyclines interfere with mitochondrial and chloroplastic protein synthesis in plants [48]. Fluoroquinolones, in particular, inhibit DNA synthesis in eukaryotes, disrupt plastid replication, and negatively affect photosynthesis and plant morphology. Enrofloxacin and sulfadimethoxine significantly reduce plant growth, while ciprofloxacin impairs photosynthesis and plant growth. Streptomycin inhibits chlorophyll synthesis in barley (Hordeum vulgare), and tetracyclines can cause phytotoxic effects such as growth inhibition and chromosomal abnormalities. Although beta-lactams are less toxic, they can affect plastid division in lower plants [49, 50]. Additionally, tetracyclines, cephalosporins, and penicillin can disrupt the electron transport chain in plants.
Nowadays a serious threat to public health is antimicrobial resistance which is mainly due to the spread of antibiotic resistance genes (ARGs) [51, 52]. Bacteria acquire these genes and develop resistance towards antibiotics. These genes are widely distributed in the environment and can be transferred within the microbial community [51, 53]. There are various mechanisms via which these genes aid the bacteria to resist against the antibiotics (such as tetracyline, chloramphenicol, sulfonamide, β-lactams, fluoroquinolone, etc) which includes reducing the concentration of antibiotics in the intracellular region via efflux pumps, hydrolysis to inactivate antibiotics, and altering the target site of drugs [52, 54]. The various mechanisms via which bacteria develop resistance once antibiotics enter their cells is depicted in Fig 1. The dissemination of ARGs have increased due to the various anthropogenic activities such as overuse of antibiotics in human medicines, agriculture and animal husbandry [55,56]. ARGs are also among emerging contaminants that are widely distributed in soil, water and air [52]. ARGs are abundant in antibiotic rich areas such as hospitals, farms and antibiotic contaminated water [57]. It is well established fact that the amount of ARGs increases with the increase in anthropogenic activities [58]. The ARGs can be transferred from antibiotic resistant bacteria in the environment to the clinical pathogens horizontally thereby aggravating the potential risk to human health [59]. Therefore, it is crucial to assess the risk associated with ARGs and it has become a global problem. The two most common methods that are commonly used to detect the composition and the abundance of ARGs are metagenomic analysis and quantitative PCR [60, 61]. ARG profiles can be identified by analysing metagenomic sequencing data and comparing it to established reference databases. This is typically done using sequence alignment tools like BLAST against databases such as the Antibiotic Resistance Genes Database (ARDB), the Comprehensive Antibiotic Resistance Database (CARD), and the Structured Antibiotic Resistance Gene (SARG) database [52]. These resources enable the detection and classification of various resistance genes present in environmental or clinical samples. Alternatively, quantitative PCR particularly high-throughput quantitative PCR (HT-qPCR)—offers advantages such as higher sensitivity, lower cost, minimal sample input, and the capability for absolute quantification of target genes [52]. Nature-based solutions, such as constructed wetlands, and phytoremediation, can effectively reduce the spread of ARG in the environment. These eco-friendly approaches enhance microbial degradation, adsorption, and natural filtration processes to limit ARG persistence and transfer.
5. Nature-Based Strategies for Remediation of Antibiotic Pollution in Wastewater
Nature-based strategies for the remediation of antibiotic pollution in wastewater leverage the inherent capabilities of natural systems to degrade and remove contaminants sustainably. Constructed wetlands, phytoremediation, and microbial degradation are key approaches in this domain. Constructed wetlands utilize plants, soil, and microbial communities to filter and break down antibiotics through processes such as adsorption, plant uptake, and microbial activity. Phytoremediation employs specific plants known for their ability to absorb, concentrate, and detoxify pollutants, including antibiotics, from water and soil. Microbial degradation harnesses the metabolic activities of bacteria and fungi to biologically degrade antibiotics, converting them into harmless byproducts. These strategies are cost-effective, energy-efficient, and environmentally friendly, providing sustainable solutions to mitigate antibiotic pollution while enhancing biodiversity and ecosystem health. Several nature-based strategies and their advantages are presented in Fig 2. An analytical comparison of various Nature-Based Strategies for the removal of antibiotics from contaminated water is presented in Table S2 of supplementary materials.
5.1. Constructed Wetlands
Constructed wetlands serve as one of the cost-effective methods for the remediation of antibiotics [62, 63]. It involves various natural processes such as biodegradation, adsorption, volatilization, hydrolysis, and photodegradation for the treatment of contaminants [62]. All the processes involved are depicted in Fig 3. They offer several advantages such as low operational costs, ease of operation, and high removal efficiency [64, 65]. On account of these advantages, they have been used recently for the degradation of several types of contaminants including antibiotics [65]. One of the recent studies demonstrated the removal efficiency of about 90% for antibiotics by using constructed wetlands [62]. The major mechanisms involved were adsorption, biodegradation, and uptake by the plants. One of the most crucial factors that governs the degradation process is the type of substrate as it is known to influence the environmental conditions where the process of treatment occurs. Gravel, clay, calcite, sand, marble, slag, fly ash, bentonite, and vermiculite are the various substrates that are commonly employed in constructed wetland systems [62, 63].
Choi et al. [66] evaluated the potential of microcosm-constructed wetlands for the removal of different classes of antibiotics such as sulphonamides and tetracyclines. The study demonstrated the efficacy of constructed wetlands in removing antibiotics from livestock wastewater with average removal efficiencies of 85% for sulfamethazine, 81.86% for sulfathiazole, 49.43% for sulfamethoxazole, 29.47% for chlortetracycline, and 27.26% for enrofloxacin, while trimethoprim, tetracycline, and oxytetracycline were minimally removed. It was observed that the removal efficacy was more pronounced for sulphonamides as compared to tetracyclines due to their higher pKa values, which resulted in enhanced adsorption into negatively charged soil via electrostatic interaction. Lab-scale microcosm studies revealed that biodegradation and direct adsorption into soil and plants are major removal mechanisms, especially for sulphonamide antibiotics.
In another study, the performance of mesocosm scale constructed wetlands having hybrid designs and artificial aeration was evaluated for the elimination of antibiotics [67]. Hybrid-constructed wetlands with zeolite as substrate and Iris tectorum Maxim as a plant material showcased the removal efficiency in the range from 87.4% to 95.3% for the removal of antibiotics. Microbial degradation played a significant role in the degradation of antibiotics.
Constructed wetlands operating at two different hydraulic retention rates of 1 and 3 days respectively were utilized for the removal of antibiotics present in the effluents of wastewater treatment plants [68]. Both constructed wetlands displayed high removal rates ranging from 28 to 100% for antibiotics. The removal efficiencies of constructed wetlands were influenced by several physicochemical characteristics whereas it was independent of retention rates.
One of the significant sources of antibiotic pollution is discharge from aquaculture. The study by [69] demonstrated the potential of constructed wetlands with different substrates, plant material, and hydraulic retention times for the removal of antibiotics present in aquaculture discharge. The maximal removal efficiencies for various antibiotics like trimethoprim, sulfamethoxazole, sulfamonomethoxine, sulfamethazine, and sulfadiazine were found to be (89 ± 3%), (61 ± 7%), (20 ± 8%), (20 ± 9%), and (12 ± 13%) respectively. Microbial degradation was the primary pathway involved in the removal of antibiotics. The removal efficiency of constructed wetlands was influenced by hydraulic retention times, substrates, and plant materials that may alter the microbial communities. Anaerobic degradation of antibiotics was carried out by bacteria such as Lacihabitans and Ilumatobacter and this degradation was inhibited by aerobic bacteria like Hydrogenophaga and Pseudomonas.
On account of advantages such as high removal efficiencies and low energy consumption, bioelectrochemistry-integrated constructed wetlands are widely utilized for the removal of antibiotics. The performance of microbial fuel cell integrated constructed wetlands and direct current integrated constructed wetlands were evaluated for the simultaneous removal of antibiotics and nitrogen by [70]. The maximal removal of 99% was achieved for ofloxacin. The major mechanism involved was found to be biodegradation. As compared to direct current bioelectricity was found to enhance microbial diversity in constructed wetlands. Another study utilizing microbial fuel cell-constructed wetlands with two different substrates (coke and quartz sand) for the removal of antibiotics and nitrogen was conducted by [71]. The constructed wetland with coke as a substrate demonstrated a removal rate of 93.60% and 70.29% for sulfamethoxazole and nitrogen respectively. More electricity was generated in wetlands having coke as a substrate. Firmicutes (18.56–30.82%), Proteobacteria (23.33–45.76%), and Bacteroidetes (17.1–27.85%) were the major microbial phyla responsible for removing antibiotics.
The removal efficiencies of vertical down-up flow constructed wetlands for antibiotics, oxytetracycline, and sulfamethoxazole were evaluated [72]. The results of the study indicated a high removal efficiency of (84.3 ± 3.5%) and (77.7 ± 3.9%) for oxytetracycline and sulfamethoxazole respectively. Various removal pathways involved were found to be microbial degradation, plant absorption, photodegradation, and substrate adsorption. Another study reported the efficient removal of antibiotics tetracycline and chloramphenicol utilizing constructed wetlands having macrophytes like Lemna gibba and Azolla filiculoides, substrates like sand and silt, and constructed wetland with or without fuel cells [73]. The study demonstrated the high removal efficiency of 100% for both oxytetracycline and sulfamethoxazole antibiotics. Table 1 summarizes the different studies, the antibiotics investigated, the types and configurations of constructed wetlands used, the removal efficiencies achieved, the major removal mechanisms identified, and key findings from each study.
5.2. Phytoremediation
Phytoremediation stands out as an environmentally sustainable method for the removal of antibiotics from various ecosystems. This process involves the use of plants to detoxify and eliminate harmful xenobiotics through several mechanisms: adsorption, uptake, translocation, metabolism, and storage of contaminants [74]. Traditionally, phytoremediation is implemented in terrestrial environments, where plants either passively or actively absorb contaminants from the soil. Various phytoremediation approaches have been depicted in Fig 4. However, this process is not limited to soil; it can also be effectively carried out in aquatic settings through the cultivation of hydroponic plants [75]. Plants are capable of transforming organic pollutants into less harmful or non-toxic compounds [76], thereby reducing the risk of secondary pollution more effectively than other remediation methods. In cases where pollutants excessively accumulate within plants, regular harvesting can be employed, and secondary pollution can be managed through composting. Thus, phytoremediation is widely recognized as an eco-friendly, in situ remediation technology, praised for its cost-effectiveness, efficiency, and environmental benefits [75].
In addition to the direct action of plants, the microbiota associated with them play a crucial role in their overall health and effectiveness in phytoremediation. These microbial communities are integral to the plants’ physiological processes, aiding in colonization and enhancing fitness [77]. By examining the relationship between specific microbial taxa and their genes, scientists can gain deeper insights into how these interactions contribute to the success of phytoremediation efforts. This symbiotic relationship between plants and their microbiota underscores the complexity and efficacy of phytoremediation as a method for removing antibiotics and other contaminants from the environment.
Phytoremediation utilizes plants to remove contaminants like antibiotics from the environment. One important way that pollutants are absorbed by plants through their root systems is through plant absorption. This process, which may involve both passive and active transport mechanisms, allows antibiotics and other contaminants to enter the roots and travel through the vascular system [74]. Once inside the plant, the contaminants may accumulate in the roots, stems, or leaves, among other areas. The ability of plants to bioaccumulate pollutants in their tissues is essential for the removal of pollutants from contaminated soil or water [78]. Another significant mechanism is rhizodegradation, which occurs in the rhizosphere, the area surrounding plant roots [79]. Plants release organic compounds, such as sugars, amino acids, and other organic materials, through their roots. The growth of soil microbes that can degrade pollutants is promoted by these root exudates [80]. These plant root exudates increase the efficiency of antibiotic breakdown by accelerating the microbial breakdown of pollutants in the rhizosphere [81]. The relationship between soil bacteria and plant roots is crucial for the removal of organic pollutants such as antibiotics. Lastly, enzymatic activity is another important phytoremediation technique [82]. Plants can produce a variety of enzymes that help break down pollutants, such as laccases and peroxidases [83]. These enzymes take part in oxidative reactions that degrade complex organic materials, including drugs and antibiotics. By hastening the breakdown of pollutants, plant enzymes contribute to the overall detoxification process and enhance the plant’s ability to purify contaminated environments [74].
There are numerous factors which influence the phenomenon of phytoremediation that are discussed below:
5.2.1. Plant species
Different species of plant show different removal rates for antibiotic remediation. Plants with different life forms such as floating, submerged, and emergent significantly increases antibiotic removal when compared to controls. Canna indica, Ipomoea aquatica, Salvinia moleata, Lemna minor, Azolla filliculoides, Eichhornia crassipes, and Egeria densa showed significant removal rates (60–95%), whereas other species, like Myriophyllum aquaticum and Acorus calamus, had no effect. In one of the studies, it was found that Canna indica demonstrated higher removal rate for antibiotics as compared to Acorus calamus [84]. Similarly another study reported that higher removal rate for sulphonamides was achieved by Cyperus papyrus as compared to other plants [85]. The variation is linked to rhizosphere microbial activity, plant species, and root biomass. Plants with larger root systems have more surface areas and can support more microorganisms, which improves antibiotic absorption and biodegradation [84, 85].
5.2.2. Type of antibiotics
Phytoremediation is also influenced by the type of antibiotics used. The removal of macrolides, β-lactams, and aminoglycosides was less effective by phytoremediation due to a lack of research than the removal of quinolones (83.17%), tetracyclines (68.25%), and sulphonamides (99.19%). This diversity is influenced by the physicochemical properties of antibiotics, such as their hydrophobicity, solubility, structure, and molecular weight, all of which affect plant uptake [86, 87]. Antibiotics with a molecular weight of less than 1000 g/mol and a log Kow between 0.5 and 3 are easier for roots to absorb [60].
5.2.3. Contact time
The removal efficiency of antibiotics by phytoremediation process is highly dependent on time. The major mechanism involved in the phytoremediation include adsorption, plant uptake and microbial degradation. First, because of its quick action, adsorption is the primary route for antibiotic elimination in phytoremediation. Adsorption slows down with time as active sites become scarcer, and microbial breakdown and plant uptake take over. Usually, the removal procedure proceeds according to first- or zero-order kinetics [85, 86].
5.2.4. Concentration of antibiotic
Plant adsorption rises with antibiotic concentration, and equilibrium takes longer to achieve. A larger concentration gradient promotes diffusion, which increases removal efficiency at higher doses because antibiotics are primarily absorbed by passive transport [75, 90].
Numerous studies have demonstrated the ability of plants to effectively remove antibiotics from water and soil environments [91–93]. The potential of phytoremediation for antibiotic removal has been well-established. However, the efficiency of antibiotic removal is influenced by various factors [94], including plant species [95], types of antibiotics, and nutrient concentrations [96].
Advancements in Next Generation Sequencing technology have significantly enhanced microbial ecology, enabling the identification of antibiotic-resistant genes within microbiomes [97]. This knowledge deepens our understanding of the molecular mechanisms behind antibiotic degradation and facilitates the discovery of novel microbial strains. By increasing the abundance and diversity of bacteria with high enzyme activity and pollutant-degrading potential [98], we can improve phytoremediation processes. Many studies provide quantitative insights into plant-microbiome interactions during antibiotic degradation, and beneficial microbiomes can be introduced into the rhizosphere to support plant growth. Efficient plant growth-promoting rhizobacteria can mitigate abiotic and biotic stress on plants. Additionally, engineering plant root exudates can further enhance antibiotic degradation and phytoremediation efficiency [99]. Table 2 summarizes the brief overview of various organisms and their efficacy in antibiotic removal.
5.3. Microbial Degradation
The removal of antibiotics via microbial degradation is becoming increasingly significant. This process is influenced by various factors, including the types of microbial species involved, the presence of anaerobic or aerobic conditions, the availability of other easily biodegradable substrates, the concentration of the antibiotics, precipitation, and temperature [41, 106]. The process of utilizing microorganisms for the breakdown of complex chemical substances present in soil and water into simpler molecules is known as bioremediation or biodegradation. This process offers several advantages over the conventional treatment methods such as cost-effectiveness, and environmental friendliness. The process of bioremediation can occur in ex-situ or in-situ mode. In the in-situ bioremediation process, the contaminated soil or water is treated at their original site whereas ex-situ bioremediation involves the removal of contaminated soil and water from their original site and are treated elsewhere. The in-situ process is cost-effective and is more environmentally friendly as compared to the ex-situ process [38, 107].
Microorganisms break down pollutants like antibiotics through a variety of mechanisms, including microbial metabolic pathways and enzymatic breakdown [108]. The mechanism of microbial degradation of antibiotics is depicted in Fig 5. One primary method is the secretion of extracellular enzymes by microorganisms. These enzymes, which include oxidases, dehydrogenases, and hydrolases, break down the chemical structures of pollutants and render them non-toxic [109, 110]. Laccases and peroxidases are examples of microbial enzymes that have the ability to oxidize complex organic compounds, including antibiotics [111]. By mineralizing pollutants or oxidizing them into less hazardous forms, these enzymes can aid in their removal from the environment. In addition to enzymatic breakdown, microbes also use specific metabolic pathways to break down organic pollutants [112]. In the case of antibiotics, fungi and bacteria have evolved metabolic enzymes that hydrolyze, reduce, or oxidize compounds such as macrolides, sulfonamides, and tetracyclines [113]. These processes break down the pollutants into smaller, less harmful byproducts, which helps with detoxification. Furthermore, co-metabolism is an additional microbial process that occurs frequently in the presence of other nutrients and involves microorganisms breaking down antibiotics as a secondary substrate rather than as a primary energy source [114]. This process enhances the microbial breakdown of pollutants and is aided by enzymes that can break down a broad range of organic molecules.
There are numerous factors which influence the phenomenon of microbial degradation that are discussed below:
5.3.1. pH
Microbial activity is highly pH-dependent because the majority of bacteria grow best in a neutral pH range (roughly 6–7) [115]. Degradation of pollutants may be slowed by deviations from this range. Alkaline or acidic environments that inhibit microbial enzymes or disrupt microbial metabolic pathways can decrease degradation efficiency [116].
5.3.2. Temperature
Temperature is crucial for microbiological degradation. High or low temperatures may have contradictory effects because most microorganisms prefer a specific temperature range [117]. Low temperatures can slow down microbial activity, while high temperatures can increase metabolic rates but also denaturize enzymes or even kill microbial cells, which slows down the rate of breakdown [118].
5.3.3. Substrate concentration
The concentration of pollutants, such as antibiotics, is a crucial factor in microbial breakdown. While low to moderate amounts can typically be efficiently digested by microorganisms, high concentrations may have inhibitory effects or cause toxic damage to microbial cells, slowing down the rate of breakdown [119].
5.3.4. Retention Time
The retention period, or the duration of the microorganisms’ interaction with the contaminated environment, has a direct impact on microbial degradation [120]. Longer retention periods allow for more sustained microbial activity, allowing microorganisms to effectively break down pollutants. If the retention time is too short, the microorganisms may not have enough time to fully degrade the pollutants.
5.3.5. Oxygen and nutrient availability
Adequate oxygen levels are necessary for the growth of aerobic bacteria, which are vital for the degradation of organic contaminants. Furthermore, the availability of essential nutrients like carbon and nitrogen may encourage microbial proliferation and improve the efficiency of antibiotic degradation by providing microorganisms with the resources they require for metabolism [121].
Microorganisms are known to degrade various pharmaceuticals including paracetamol, ibuprofen, and various antibiotics. Microbes have the potential to degrade fluoroquinolones that cannot be easily degraded by other conventional treatment methods such as aerobic and anaerobic methods [122, 123]. Significant degradation of antibiotics such as norfloxacin, ciprofloxacin, and ofloxacin with a concentration of 10 mg/L was achieved by using a combined consortium of microbes (Labrys portucalensis F11, Rhodococcus sp. FP1, and Rhodococcus sp. S2) [124]. The study documented the ability of microbes to degrade the fluoroquinolones at higher concentration levels.
Another study reported the use of a bacteria isolated from sludge namely, Thermus thermophilus C419 for the degradation of fluoroquinolones. 51.45% of ciprofloxacin was degraded at a concentration of 5mg/L after 120 hours of incubation with bacterial strain. Additionally, other fluoroquinolones such as enrofloxacin, oxfloxacin, and norfloxacin were degraded after 72 hours of incubation with the strain C419. The removal rate was 74, 70, and 63% for enrofloxacin, ofloxacin, and norfloxacin respectively [122].
Microbes have the potential to degrade the sulfonamides also [125, 126]. Acinetobacter and Pseudomonas bacteria were used to degrade 99.8% sulfamethoxazole, 96.6% sulfadimethoxine, and 97.8% sulfamethazine on the 82nd day of incubation. Moreover, several other bacterial strains isolated from sludge like Arthrobacter, Paracoccus, Methylobacterium, and Kribbella are known to degrade sulfadiazine. The degradation rate varied from 50 to 99.8% depending upon the type of microbial strain utilized for degradation [125].
A mixed consortium of Bacteroidia and Proteobacteria was utilized for the degradation of ciprofloxacin [127]. Ciprofloxacin degradation products are usually formed via dealkylation, deamination, and the cleavage of C–F bonds. Another study reported the degradation of sulfonamides in natural sludge systems by microbes which showed acetylation, hydroxylation, and oxidation processes were mainly involved in the degradation process [128]. Microbial species Firmicutes, Proteobacteria, Bacteroidetes, and Acidobacteria are known to degrade sulfamethazine. The bacterial species Bacteroidetes, Chryseobacterium, and Firmicutes have the potential to degrade sulfanilamide. Microbial species such as Acinetobacter, Rhodopirellula baltica, Micrococcus luteus, Delftia acidovorans, Oligotropha carboxidovorans, Pseudomonas sp., and Methylibium petroleiphilum have the ability to degrade the sulfonamides. A 100% degradation rate was achieved for the removal of sulfamethoxazole by microorganisms [38, 129]. Since bacteria can survive in extreme conditions and they possess the inherent ability to thrive in stressed environments, they are primarily utilized in the process of bioremediation which is an effective method for the degradation of antibiotics present in the environment. Table 3 depicts various microorganisms and their efficiency in the removal of antibiotics.
6. Research Gaps
Despite growing interest in Nature-Based Solutions (NBS) for antibiotic removal, several critical research gaps remain:
The microbiological, chemical, and physical processes that promote the breakdown, transformation, or uptake of antibiotics and resistance genes inside NBS systems (such as constructed wetlands and phytoremediation) are not well understood [134].
The majority of studies are site-specific and lack long-term performance data under various operational and climatic conditions, as well as standardized methodologies. Comparative assessments between NBS types and traditional treatment systems are rarely conducted [135].
The majority of current research is conducted at the lab or pilot stage. There aren’t many field-scale installations with reliable monitoring, which limits how far the findings may be applied.
There is a lack of standardized indicators and metrics to evaluate the multifunctionality and co-benefits of NBS.
7. Challenges, Opportunities and Future Perspectives of Implementing Nature Based Solutions
The implementation of NBS for the remediation of antibiotic pollution from wastewater presents several challenges that must be addressed to achieve widespread adoption and effectiveness. One significant challenge is the scientific and technical limitations inherent in understanding and optimizing these solutions. The complex interactions within ecosystems and the specific conditions required for the optimal performance of NBS, such as phytoremediation, necessitate comprehensive scientific research and technical expertise. Additionally, economic constraints can hinder the initial adoption of NBS, despite their long-term cost-effectiveness. Securing adequate funding and demonstrating the economic viability of these solutions is particularly challenging in developing regions where financial resources are limited. Policy and regulatory hurdles further complicate the implementation of NBS. Existing regulatory frameworks often do not accommodate these innovative approaches, and there is a need for supportive policies that facilitate their integration. Bureaucratic inertia and a lack of political will can impede the necessary policy reforms. Furthermore, public perception and awareness are critical to the success of NBS projects. Limited understanding and misconceptions about the benefits of these solutions can result in insufficient community support and engagement, which are essential for the long-term sustainability of NBS initiatives.
Despite these challenges, the opportunities presented by NBS for the remediation of antibiotic pollution are substantial. NBS offers multifunctional benefits that extend beyond pollution control, including biodiversity enhancement, improved air quality, and carbon sequestration. These co-benefits can make NBS more attractive compared to conventional engineering solutions. Additionally, NBS provides opportunities for meaningful community involvement and empowerment, fostering education, stewardship, and local employment. This can strengthen community resilience and social cohesion, creating a supportive environment for sustainable practices. The cost-effectiveness of NBS over time is another significant opportunity. Unlike conventional methods, NBS often requires less maintenance and can provide ongoing benefits without significant additional investments. Advances in innovative research and technologies, such as remote sensing, geographic information systems (GIS), and bioengineering, are further enhancing the design, monitoring, and management of NBS, making them more efficient and effective.
Looking to the future, integrated approaches that combine ecological, social, and technological perspectives will likely enhance the resilience and multifunctionality of NBS. Strengthening policy frameworks and institutional support will be crucial for scaling up these solutions, including creating incentives for private sector participation and integrating NBS into national and international environmental policies. Enhanced monitoring and evaluation frameworks are essential to assess the performance and benefits of NBS, helping to refine these solutions and demonstrate their value to stakeholders. NBS also holds significant potential for climate change adaptation and mitigation. Future research should focus on optimizing these solutions to enhance their effectiveness in sequestering carbon, reducing greenhouse gas emissions, and improving climate resilience. Finally, global collaboration and knowledge sharing can accelerate the adoption and improvement of NBS. International platforms and networks can facilitate the exchange of best practices, innovations, and lessons learned, ensuring that NBS is effectively implemented worldwide. In conclusion, while the implementation of NBS for the remediation of antibiotic pollution from wastewater faces several challenges, the opportunities they present for sustainable development, environmental protection, and community resilience are immense. By addressing the challenges and leveraging the opportunities, NBS can become a cornerstone of future environmental and societal strategies.
8. Conclusion
Antibiotic pollution in wastewater has emerged as a significant environmental challenge, with far-reaching implications for ecosystems and public health. This review has elucidated the origins and pathways of antibiotics into aquatic systems, highlighting their diverse sources and the complexities of their environmental dynamics. The accumulation of these contaminants in freshwater sources and wastewater exacerbates environmental degradation, disrupting the natural balance and posing severe risks to biodiversity and ecological health. NBS has demonstrated promising potential in addressing this pressing issue. By leveraging strategies such as constructed wetlands, phytoremediation, and microbial degradation, NBS offers a sustainable and cost-effective approach to mitigating antibiotic pollution. These solutions harness the inherent capabilities of natural processes to degrade and remove contaminants, promoting environmental well-being and safeguarding human health. However, the implementation of NBS is not without challenges. Scientific and technical limitations, economic constraints, policy and regulatory hurdles, and public perception issues must be navigated to achieve widespread adoption and effectiveness. Addressing these challenges requires a multidisciplinary approach, integrating comprehensive scientific research, supportive policy frameworks, adequate funding mechanisms, and robust community engagement. Future research should focus on optimizing NBS techniques, understanding their long-term impacts, and developing innovative solutions to enhance their effectiveness. By advancing our knowledge and addressing the barriers to implementation, we can better harness the power of nature to combat antibiotic pollution. Ultimately, the successful integration of NBS will contribute significantly to environmental sustainability, protecting both ecosystems and public health for generations to come.
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