## Introduction

In recent centuries, our environment has become a dumping ground of natural and synthetic chemicals, such as pharmaceuticals and personal care products (PPCPs), and active pharmaceutical ingredients or pharmaceutically active compounds1,2,3. Among various PPCPs, antibiotics (ABs) are becoming ubiquitous in the environment and pose serious human and environmental health concerns1,2. ABs are classified into various types based on their chemical structure and mechanisms of action (i.e., aminoglycosides, penicillins/beta-lactams, quinolones, tetracyclines, macrolides, lincosamides, and sulfonamides). Moreover, they can also be grouped into two classes based on their spectrum known as narrow-spectrum ABs (i.e., narrow-spectrum penicillins) and broad-spectrum ABs (i.e., broad-spectrum penicillins). A summary of their basic structure, chemical formulas, molecular weight, and trade names is provided in Table 1. Growing demands and extensive application of ABs in various sectors lead to their massive annual production and consumption (131,000 tons used only for animal production globally in 2013) across the globe (Table 1). Recently, AB prevalence in the environment is being noticed and causing concern due its wide applications in farming, aquaculture, human, and animal illnesses2,3,4, animal husbandry, and livestock storage for prophylactic, therapeutic, and growth promotor agents5,6. Considering that there are only limited studies in emerging economies, we have explicitly mentioned India’s percentage share of India in the global consumption of ABs (Table 1).

Due to the extensive application and wide-spectrum use of ABs in recent decades, AB residues are now frequently being detected in raw or treated wastewater (WW)7,8,9, hospital effluents2,10,11, surface water2,12,13,14, groundwater15,16, and soil sediments in the natural systems6,17. The range of concentration is from ng L−1 (norfloxacin, 11.1–964 ng L−1)8 to mg L−1 (ciprofloxacin, 28–31 mg L−1)18. Owing to the reality that ABs can cause severe damage to the aquatic and human life even in trace concentrations, their buildup in the environment and way up to the trophic levels has been recognized as one of the gravest environmental issues to policymakers and industry personnel worldwide16. Due to tedious analytical procedures for the finding of AB concentration, only a few studies in the emerging economies, suh as India, Pakistan, Bangladesh, and Srilanka, have reported19 in the removal methods7,20, and the impact of ABs on aquatic matrices3,4. Thus, it is imperative to compare the prevalence of ABs in various parts of the world and from different aqueous matrices (surface water, groundwater, and WW).

At present, the common consensus is that municipal WW treatment plants (WWTPs) are not always efficient21 enough to entirely remove PPCPs from the WW1,22 and thus act as the provenance of pharmaceuticals (specifically ABs) into the environment23. There is the need of the hour to develop more efficient purging techniques, to meet the goal of WW reuse, and to meet the criteria for safe discharge into the ambient water environment24. Several treatment processes have been used for AB removal, including ozonation, Fenton oxidation (dark light)25, photo-Fenton oxidation26,27,28,29, semiconductor photocatalysis (SP) using undoped/doped titania (TiO2)30, photocatalysis24,31,32, Fenton oxidation combined with reverse osmosis (two-stage RO)33, and electrochemical oxidation34. Adsorption-based techniques using activated carbon35 or biochar36,37,38, or ecological treatment processes like constructed wetlands (CWs) and stabilization ponds1,8,39 have also been tried and tested for their efficiency on the removal of AB. Therefore, it is very pertinent to have a comparative understanding of the pros and cons of each available technique, along with the best efficiency achieved with it.

Overall, the first two decades of the twenty-first century have been the decades of water, health, and sanitation, where AB resistance in human pathogens has become a critical health and social issue. It has led to numerous research articles on the occurrence, fate, transport, and removal of ABs. However, a single review article that may kickstart with types, classes, use, and side effects of ABs, and then attempts at providing the recent proclivities of prevalence, pathways, and purging of ABs in ambient water, is of immense importance. Thus, under the light of the above discussions, this review provides a comprehensive account of the consumption pattern and reported occurrences in the environmental matrices to build a better understanding of the removal efficiencies of the existing techniques with a particular emphasis on India. The objective is threefold, namely (i) the pattern of consumption, prevalences in ambient waters, pathways, and environmental implications of various ABs, (ii) benefits and apprehensions of existing AB removal/purging techniques, and (iii) enumeration of the most frequently used removal techniques and their efficiency, kinetics, and isotherms associated with AB removal. We intend to report the decadal change in production and consumption of AB along with reported prevalence in the various aqueous matrices in the selected part of the world, and then provide the best adsorbent, most fit isotherm, and removal kinetics reported for AB purging through extensive discussion on various advance removal techniques and processes. This review tries to cover the overall cycle of AB contaminants for both the general and specialized audience working in this field.

## Pattern of consumption and prevalence of contamination

### AB consumption pattern

AB consumption is increasing on a daily basis due to increasing population, affordability, lifestyle, and availability. Figure 1a–d shows the spatial and temporal variation in AB consumption of selected countries from the developed world (Australia, Japan, Sweden, United Kingdom, and the United States) and the developing world (Bangladesh, China, India, Pakistan, and Srilanka) (the year 2000–2015). Given the reported data, it is not surprising to see the drastic increases of ABs used from 131,000 tons in 2013 and are expected to be 200,000 tons by 203040. The top five AB-consuming countries were India (2623 tons in 2013, predicted to increase by 84% by 2030), China (78,200 tons in 2013, predicted to surge by 59% by 2030), United States (9476 tons in 2013, 22% increase by 2030), Brazil (6448 tons in 2013, 41% increase by 2030), and Spain (2202 tons in 2013, 6% increase by 2030)40. The percentage share of different AB consumption in India is shown in Table 1. Penicillin is the most consumed AB followed by tetracycline, quinolones, microlides, trimethoprim, and aminoglycosides (Fig. 2). Consumption of ABs in the selected developed countries is almost 1.5–3 times higher than developing countries from the developing world. The total use of penicillin and trimethoprim in the year 2000 (Fig. 1a) in the selected countries from the developed and developing world were 12,384 and 3374, and 1705 and 1350 daily dose (DD) per 1000 people, respectively. No distinct difference in AB consumption pattern could be observed between selected countries from the developed and developing world (Fig. 1). Apart from the increased consumption of tetracycline and macrolides in India from the year 2000 to 2015 (Fig. 1a–d), no specific AB consumption pattern seems obvious.

### Prevalence of AB contamination

Table 2 and Supplementary Table 1 summarizes the AB contaminants along with their concentrations in various environmental spheres. Generally, AB concentrations are reported in mg L−1 in pharmaceutical effluents and hospital effluents11,18, in μg L−1 in municipal effluents7, and ng L−1 in freshwater (surface and groundwater) and seawater. These broad-range concentrations of ABs are found universally applicable with good spatial and temporal change around the globe41. Figure 2 depicts the concentration of ABs reported in India, China, and the United States, with the highest reported AB concentration in the Indian subcontinent, followed by China and the United States. Among all ABs, ciprofloxacin showed the highest concentration of 31,000,00018 ng L−1 in the ambient waters, which is indicative of its vast abundance and usage.

Interestingly, due to the varied types and uses of ABs, several of them were not common in various countries, making their comparison challenging. The concentration of ABs reported in the WW in the past two decades is shown in Table 2 and Supplementary Table 1. In India, the highest level of ABs reported is of ciprofloxacin (28,000,000–31,000,000 ng L−1), followed by losartan (2,400,000–2,500,000 ng L−1) and cetirizine (1,300,000–1,400,000 ng L−1)18. Another Indian study showed high levels of ampicillin (104,200 ± 98,110 ng L−1) in the WW41. It was observed that the AB concentration is higher in WW than other environmental aqueous matrices, that is, surface (river) water and groundwater and hospital effluents. Several studies reported the AB traces in the river water systems. Mutiyar and Mittal41 studied the occurrence of ABs in the Yamuna river, India. They reported the high concentrations of ampicillin (13,800 ng L−1), ciprofloxacin (1400 ng L−1), gatifloxacin (480 ng L−1), sparfloxacin (2100 ng L−1), and cefuroxime (1700 ng L−1) in river water. These concentrations of ABs are roughly a thousand times lower than the values observed in WWTP effluents (Table 2 and Supplementary Table 1)18. Varying levels of AB concentrations have been reported in water in Indian rivers: amphetamine (984 ng L−1) in Cooum river, Chennai42; sulfamethoxazole (900–1600 ng L−1) in Kaveri river42; ciprofloxacin (10–130 ng L−1) and sulfamethoxazole (50–120 ng L−1) in the Ganges River43; ciprofloxacin (7447–5,015,000 ng L−1), enrofloxacin (2262–181,609 ng L−1), norfloxacin (16,148–251,137 ng L−1), and pefloxacin (741–38,335 ng L−1) in river Mousi44. Concentrations of carbamazepine (3–75 ng L−1), theophylline (609–2939 ng L−1), and acetaminophen (2156–5967 ng L−1) were found in the Brahmaputra river, and its tributary Bharalu, Assam, India2.

Overall, the concentration range of ABs found in Indian rivers was much higher than that in China. Figure 3 shows the variation in AB concentrations reported worldwide. Table 3 shows the concentration of ABs found in different environmental spheres of India, China, the United States, and the rest of the world, indicating a significant deviation in reported AB concentrations (2000–7000 ng L−1) ranging from few ng L−1 (3.24 ng L−1) to thousands of μg L−1 (31,000 μg L−1). No correlation was found between the concentration of the ABs in different matrices within the same country.

### Impending environmental consequences of AB prevalence

The occurrence of high concentrations of ABs in the aquatic ecosystem and the resulting AB-resistant genes (ARGs) have recently been reported as a serious risk to the environment and humanoid health32. The presence of ABs in the environment has severe effects on aquatic and earthly organisms45,46,47. ABs alter the metabolic activities and community composition in microbes48, leading to environmental pollution by AB-resistant bacteria (ARB) and ARGs2,9,49,50,51. Even in low concentrations, they pose a grave environmental problem due to the high endurance and ecotoxicity, influence the aquatic species, and have severe effects on the ecosystem52. AB contamination effect includes the development of direct toxicity to microorganisms (flora and fauna) and probable risks to humanoid health through the intake of polluted non-grail fauna53.

The high concentration of ABs in the environment could act as bacteriostatic and bactericidal agents for the microbes. The bacteriostatic proctors reduce the proliferation of bacterial cells, whereas bactericidals eradicate the bacterial cells54. AB encroachment enhances the proliferation and enzymic activities of the bacterial biome (biotic community), which alters the ecological functions (i.e., biomass production and transformation of nutrients)55,56. These effects lead to changes in the transformations of nitrogen species, methanogenesis, sulfate reductions, nutrient cycles, and degradation of organic matters57. Some of the bacteria can develop resistance against the ABs, as they may have been exposed already to such types of molecules in the environment58. As summarized in Supplementary Table 2, most of the ABs affect bacterial growth and microbial biomass abundance, which may decrease or increase depending on the concentration of ABs59,60,61. The occurrence of ciprofloxacin in water may reduce the respiration or catabolic activity of microbes61. Nitrification and denitrification are also affected due to the presence of some ABs like sulfadiazine62 and sulfamethoxazole63. The occurrence of AB residues in the environment alters the natural cycles and imbalance of the whole ecological system.

### Sources and pathways of ABs

Since the discovery of ABs, their demand and production have grown exponentially worldwide, and consequently are discharged continuously into the environment as metabolites, deterioration products, or both by varied origins of input12,41, as shown in Fig. 4. The source of ABs is primarily the human/animal excretion, as almost 80–90% of consumed ABs are discharged in consistent form and metabolites, because of the partial absorption by organisms in human and animal bodies64. AB contamination spreads through WWTPs, runoff from agricultural fields, anthropogenic effluents, animal houses, slaughterhouses, and runoff from landfill34. Recently, WWTPs are considered as the primary point source of AB contamination in the aquatic environment23,65,66. However, unmanaged disposal of expired ABs from factories, hospitals, medicals, and residential locality contributes significantly67 to sewerage and WWTP systems. ABs used as fertilizers or pesticides may pollute the soil and subsequently surface water and groundwater through runoff, infiltration, or percolation after rainfall and from landfill sites3,4.

The effluent containing AB residues is generated from the residential areas (residential and hospitals), industries, slaughterhouses, animal houses, and dairy farms, and finally reaches the WWTPs. The sludge generated from the WWTPs also contains AB residues and is disposed of at landfill sites or used as a soil conditioner in the agricultural lands. Thus, WWTPs are well-known point sources, and agricultural fields are a line source of AB contamination. Wastes (liquid/solid) from pharmaceutical units or incidental spills during production and transportation can also be considered primary point sources of AB contamination in the environment68,69. Also, the direct discharge from aquaculture (fisheries), poultry production is an important pathway of AB contamination. Overall, AB contamination in the environment occurs in a closed-loop manner termed as an “AB contaminant cycle” that includes food chain routes and bioaccumulation and biomagnification through crop irrigation by WWTP effluents or contaminated soil, trophic levels, and aquaculture habitat (fish and crabs). Figure 4 also depicts how ABs enter into different ecosystems and finally reach the origin in a closed-loop manner.

## AB purging techniques and processes

### AB purging techniques

Table 4 summarizes the various techniques used for AB degradation and removal. Adsorption, SP, and ozonation techniques are the most studied techniques for AB removal. As far as the removal processes are concerned, both nonbiotic processes like sorption, hydrolysis, photolysis, oxidation, reduction, liquid extraction, and membrane techniques, and biotic processes are equally tried and tested for the purging of ABs. It has been reported that most of the removal techniques depend on factors, such as the composition of the matrix, temperature, pH of the system, and concentration of contaminants. In general, it has been reported that the conventional WWTPs fail to remove the highly polar contaminants like ABs2,3,21, which emphasizes the need to develop elimination techniques for such contaminants3,4. We present an overview of different removal techniques used for eliminating ABs in the past two decades.

### In situ techniques (CWs)

CWs are categorized under in situ removal techniques that include vegetations like Phragmites australis and Typha latifolia1 with a natural tendency to improve water quality by the cycling of nutrients and capturing contaminants from WW. In the past couple of years, researchers have been trying to find out wetlands’ efficiency to treat the AB-contaminated water1,31,35. WW treatment using CW could emerge as an economical and promising substitute to the traditional WWTPs for eliminating or reducing the level of nitrogen, phosphorus, pharmaceuticals (ABs), and biological oxygen demand (BOD) in treated water1. Earlier published research reported that CWs are very efficient in reducing the concentration of ABs and ARGs. CW could achieve relatively similar or high elimination rates of the ABs than conventional mechanized WWTPs. The finding revealed that the most frequently detected ABs in the municipal WW might be effectively reduced up to 78–100% through the CW treatment systems. The fundamental mechanisms of AB removal in CW are sorption and biodegradation70. Finding the percentage contribution of both the mechanisms remains an open question for researchers. The ABs’ removal efficiency of CW varies from 10.3% (sulfadiazine)70 to 100% (ciprofloxacin)1 (Table 4). CW could be a promising solution for AB removal from the municipal WW70, but can serve relatively small localities or towns71.

### Ex situ techniques (conventional treatment)

Conventional treatments are categorized as ex situ techniques because WW was brought to the treatment plant from the actual site and treated under various stages of filtration, coagulation–flocculation, sedimentation, and biological (activated sludge process (ASP) and trickling filters) treatment8,23,72,73. Various WW treatment processes are being tested for AB removal across the globe (Table 4). These processes work effectively for WW having low initial concentrations of contaminants. Mutiyar and Mittal41 reported that AB removal efficiency of the conventional ASP-based WWTP varies from 99.37% (sparfloxacin) to 55.47% (gatifloxacin). In China, Leung et al.23 reported that the AB removal efficiency of WWTP (ASP) varies from 70–80% (cephalexin) to 3–5% (ofloxacin). Sometimes, there is also an increase in the concentration of ABs. Prabhasankar et al.21 found that the removal efficiency varied from −23.07% (erythromycin: influent = 26 ng L−1, effluent = 32 ng L−1) to 87% (ampicillin).

The negative removal efficiencies of ABs can be explained by one of the following reasons. First, most of the ABs found that the WW samples can be excreted from the human body through the urine and feces as a mixture of stable parent compounds and conjugates of glucuronic acid74,75,76. Thus, these conjugates may undergo enzymatic cleavage to form their parent compounds throughout biological WW treatment processes, resulting in increased concentrations in treated effluents (appeared as negative removal efficiency). Second, ABs excreted through the urine and feces might be fenced in fecal particles in the WW and slowly released throughout the treatment processes, hence increasing the AB concentration in the treated effluent77 (i.e., appeared as negative removal efficiency). Third, negative removal efficiencies of ABs in the WWTPs could be due to sampling errors.

However, there is no guaranteed removal of ABs that are found in conventional WWTPs77. Several researchers studied the performance of physicochemical methods for AB removal and reported that maximum removal of 30% was achieved78. The AB removal efficiency of physical and chemical treatments depends on various physicochemical factors like effluent pH, temperature, initial concentration, and types of ABs, and operational factors of treatment system like slugde retention time and hydraulic retention time. This dependency makes removal efficiency highly variable and is a shortcoming of the conventional techniques79,80,81. Due to the low efficiencies and shortcomings of these methodologies, they are not considered feasible for AB-contaminated effluent treatment.

## Removal processes and mechanisms

Adsorption techniques are broadly practiced around the globe to treat AB-contaminated liquid waste. There are a few mechanisms involved in adsorption processes for removing the AB from the WW using adsorbents; the mechanisms are ion exchange, pore filling, and pi–pi electron interaction82, the electrostatic mechanism83, surface complexation, and hydrogen bonding. Removal using the ion-exchange mechanism depends upon the pollutant size and surface functional group (SFG) of the adsorbent used84. The pore structure of the adsorbent facilitates a AB contaminant adsorption through the pore-filling mechanism85. The adsorption efficiency is dependent on the physicochemical properties of the adsorbent as well as the medium characteristics. The characteristics of the adsorbent include surface area, porosity, and aperture opening (diameter)86, and so on. The liquid-phase characteristics are pH, temperature, type of contaminants and concentration, and organic matter.

The removal process (Table 5) is highly influenced by pH because it unswervingly affects the physicochemical properties and the adsorbent’s surface activity. Sorption reaction is temperature dependent because it is an exothermic reaction. At the same time, organic matter also influences the removal efficiency due to its direct competition with the targeted molecules (AB residues) for adsorptive sites. The nature of the adsorbent highly influences the rate of adsorption and its capacity. Several studies focused on increasing the adsorption efficiency by improving the SFG by using thermal or chemical preactivation.

Granular activated carbon (GAC) exhibited promising removal capacity with limitations about their cost and difficult synthesis procedures87. Nowadays, finding a substitute for GAC is a growing and attractive research area for the researcher. Low-cost adsorbents produced from agricultural and industrial waste, are used in the removal of ABs from WW, for example, activated carbon produced from Jerivá83, Alfalfa hays and pine wood88,89,90,91, sawdust92, rice straw and swine manure93, macadamia nut shells94, dried duckweed37, Trapa natans husk9, Albizia lebbeck seed pods93, hazelnut shell36,37,94,95, coal fly ash96, and decaffeinated tea and coffee waste88,94,97,98. The properties of these adsorbents and their removal efficiencies are summarized in Table 5.

### Oxidation

Advance oxidation treatment processes (AOTPs) result from the production of free hydroxyl and oxy radicals. These radicals are highly reactive and have an oxidation potential of 2.8 V, which is higher than ordinary or classical oxidants, making them highly effective in treating the AB-contaminated WW107. There are some oxidizing agents often used to produce hydroxyl radicals like O3, H2O2, and so on mixed with semiconductor catalysts/ultraviolet radiation/metal catalysts. In this process, it is expected that AB contaminants get oxidized and reduced into less harmful products. It might be possible that the by-products produced after oxidation are more harmful than the parental compounds.

### Ozonation

Ozone is known as an effective oxidant due to its high oxidation potential (E0 = 2.07 V). Ozone has high redox potential than classical oxidants like H2O2 (hydrogen peroxides), ClO2 (chlorine dioxides), Cl2 gas, and hypochlorites. It can react directly or indirectly with AB contaminants. The AB degradation mechanism primarily has direct oxidation rather than indirect oxidation via OH*108 (radicals). It is necessary to have a C=C (carbon–carbon) or aromatic bonds with the N (nitrogen), O (oxygen), or S (molecules of sulfur) to carry out the direct oxidation using O3 because it reacts only with the nucleophilic molecules. If this condition is not fulfilled, then the degradation of O3 in WW to generate an OH (hydroxyl radicals, it has unpaired electron) takes place through another mechanism. The reaction is given below:

$${\mathrm{O}}_3 + {\mathrm{OH}}^ - \to {\mathrm{O}}_2 + {\mathrm{HO}}_2^ -,$$
(i)
$${\mathrm{O}}_3 + {\mathrm{HO}}_2^ - \to {\mathrm{HO}}_2^ \bullet + {\mathrm{O}}_3^{ \bullet - },$$
(ii)
$${\mathrm{HO}}_2^ \bullet \to {\mathrm{H}}^ + + {\mathrm{O}}_2^{ \bullet - },$$
(iii)
$${\mathrm{O}}_2^{ \bullet - } + {\mathrm{O}}_3 \to {\mathrm{O}}_2 + {\mathrm{O}}_3^{ \bullet - },$$
(iv)
$${\mathrm{O}}_3^{ \bullet - } + {\mathrm{H}}^ + \to {\mathrm{HO}}_3^ \bullet,$$
(v)
$${\mathrm{HO}}_3^ \bullet \to {\mathrm{HO}}_ \cdot ^ \bullet + {\mathrm{O}}_2.$$
(vi)

As per reactions (i) and (ii), the degradation is pH sensitive, and hence we can accelerate the initiation of degradation by increasing the pH of the medium

$${\mathrm{HO}}_ \cdot ^ \bullet + {\mathrm{O}}_3 \to {\mathrm{HO}}_2^ \bullet + {\mathrm{O}}_2.$$
(vii)

The rate of the above reaction (vii) is fast, and it helps to minimize the oxidation capacity of the matrix, as it requires low dissolved organic carbon and alkalinity, and improves the efficiency of the treatment process. The AOTPs are very effective when the variability in inflow and AB composition in WW is high. In normal circumstances, these processes may be uneconomical due to the costly treatment units, maintenance, and energy requirement. There is a limitation related to the mass transformation in the oxidation process with O3; if the mass transformation is inadequate, it is also one of the major disadvantages. It can negatively affect the removal efficiency of the process and, hence, result in increasing the operating costs. The performance of this process is susceptible to some factors like the concentration of solids, the presence of microorganism/pathogens, CO3 (carbonate), HCO3 (bicarbonate), HRT, ozone dose109, pH, and temperature110,111.

Iakovides et al.109 have studied the removal of ABs by ozonation. They suggested that treatment optimization is necessary because excess ozone creates problems for useful microorganisms in treated effluents. Ozonation is highly dependent on the ozone dose and reaction time. A rise in the oxidant level leads to a multiplication of AB degradation rate in the treatment process. Ben et al.112 integrated the sequencing batch reactor (SBR) with ozonation and achieved the removal of ABs (tetracycline, microlide, and sulfonamides) by 80–100% efficiency. Alsager et al.113 studied AB removal (amoxicillin, doxycycline, ciprofloxacin, and sulfadiazine) from water and dairy milk, and the efficiency was 95%. Ozonation with an adequately optimized dosage is a favorable technique for removal (>99% efficiency) of AB contaminants109. Nowadays, ozonation is coupled with classical or advanced treatment processes to achieve the effective removal of ABs. Balcıoğlu and Ötker114 reported that hybrid ozone–H2O2 treatment has higher AB removal efficiency than ozone alone. Li et al.115 combined ozonation with biologically activated carbon filtration and the sponge membrane bioreactor with ozonation to remove the different AB contaminants from water.

### Photocatalysis

SP techniques have gained significant interest due to being cost-effective, ecofriendly, and known to be a sustainable technology with negligible waste production116. SP requires three components to carry out the degradation of the ABs using an oxidative mechanism, a catalyst with photosensitive (e.g., TiO2 or inorganic semiconductors), a photon energy source, and an efficient oxidizing agent22,24,30. This fundamental principle includes the stimulation of a semiconductor (generally used TiO2 because of its immense stability, efficient performance, cost-effectiveness, and ease of availability) by artificial means. The semiconductors are characterized by their valence and bandgap, and conduction bands. The mechanisms involved in this process are as follows:

$${\mathrm{Photocatalyst}}\,\left( {{\mathrm{TiO}}_2} \right) \to {\mathrm{Photocatalyst}}\,({\mathrm{e}}^ - + {\mathrm{h}}^ + ),$$
(viii)
$${\mathrm{h}}^ + + {\mathrm{contaminants}} \to _ \cdot ^ \bullet {\mathrm{contaminants}},$$
(ix)
$${\mathrm{h}}^ + + {\mathrm{OH}} \to {\mathrm{OH}}^ \bullet.$$
(x)

Reaction (viii) shows the loss of an electron through direct oxidation or photocatalysis. Reaction (ix) shows indirect oxidation. The mechanism shown in reaction (ix) is as fast as reaction (viii). All the reported studies show that for the photomineralization, the presence of H2O and molecular oxygen is necessary. The electron holes shown in Eq. (viii) have high oxidation potential to produce OH (hydroxyl radicals) from the H2O molecules/OH adsorbed over the surface of semiconductors:

$${\mathrm{TiO}}_2\left( {{\mathrm{h}}^ + } \right) + {\mathrm{H}}_{2}{\mathrm{O}} \to {\mathrm{TiO}}_2 + {\mathrm{OH}}^ \bullet + {\mathrm{H}}^ +,$$
(xi)
$${\mathrm{TiO}}_2\left( {{\mathrm{h}}^ + } \right) + {\mathrm{OH}}^ - \to {\mathrm{TiO}}_2 + {\mathrm{OH}}^ \bullet.$$
(xii)

The generated e (electrons) can decrease dissolved oxygen and produce O2 that is transformed into H2O2 (refer reactions 13–15):

$${\mathrm{TiO}}_2\left( {{\mathrm{e}}^ + } \right) + {\mathrm{O}}_2 \to {\mathrm{TiO}}_2 + {\mathrm{O}}_2^{ \bullet - },$$
(xiii)
$${\mathrm{O}}_2^{ \bullet - } + {\mathrm{H}}_{2}{\mathrm{O}} \to {\mathrm{HO}}_{2}^ \bullet + {\mathrm{HO}}_ \cdot ^ -,$$
(xiv)
$$2{\mathrm{HO}}_2^ \bullet \to {\mathrm{H}}_{2}{\mathrm{O}}_2 + {\mathrm{O}}_2^ \bullet.$$
(xv)

H2O2 similarly acts like an electron (e) receptor producing additional HO ions:

$${\mathrm{TiO}}_2\left( {{\mathrm{e}}^ - } \right) + {\mathrm{H}}_{2}{\mathrm{O}}_2 \to {\mathrm{TiO}}_2 + {\mathrm{HO}}_ \cdot ^ - + {\mathrm{HO}}_ \cdot ^ \bullet.$$
(xvi)

Earlier studies reported that decomposition of ABs not only occurs due to OH via a similar process but also other species of radicals that are derived from oxygen itself. The efficiency of the entire AB removal mechanism is also influenced by pH, temperature, types of catalyst, and its dose, the intensity of radiation, and the composition of WW. Belhouchet et al.24 investigated the applicability of SP techniques to remove the residue of tetracycline and found a wide range of removal efficiency 41.9–90.6% (at an initial concentration of 50 mg L−1). Cao et al.22 also studied the removal of tetracycline by employing SP techniques and used a hybrid catalyst, which is made up of magnetic graphene oxide cerium-doped TiO2 to accelerate the decomposition of the contaminant. Dimitrakopoulou et al.30 investigated the efficiency of SP techniques for the removal of amoxicillin ABs and used different doses of TiO2 catalyst where it was found that 250 mg L−1 was the optimum dose for maximum removal. Elimination of tetracycline was also attempted and inferred by several studies30,39. The SP technique is a favorable technology for the removal of ABs at a laboratory scale from several decades20. The SP technique is not feasible for industries because it is hard to pass radiation through liquid waste, which carries fine colloidal particles in suspensions, and challenging to eliminate the used catalyst after treatment.

### Membrane techniques

Membrane techniques come under the physical treatment process. Water is passed through a semipermeable membrane with the help of mechanical forces (pressure is applied). This technique does not degrade the ABs, but retains them over the surface or transfers them into a new phase. The pores in the membranes vary from 0.001 to 0.02 μm. Several membrane techniques are used to treat the WW, such as RO (forward, reverse, and backward), ultrafiltration, nanofiltration, and microfiltration. The RO and nanofiltration techniques as a tertiary treatment step are very effective in treating the AB-contaminated water117, but the energy requirement is too high. Other associated problems include clogging due to mudball formation, fouling of the membrane, operation low mass flux, and high capital expenditure and operating expenditure. Liu et al.118 studied the performance of a hybrid carbon membrane (graphene oxide and activated carbon) for removing tetracycline ABs from water and reported 98.9% removal with 414 m2 g−1 of surface area. They concluded that the hybrid membrane process is more effective than a single membrane. Acero et al.117 worked on the treatment of flumequine and sulfamethoxazole using nanofiltration and ultrafiltration. They achieved >70% AB removal efficiency using nanofiltration, which was higher than ultrafiltration.

### Procuring prospective

Many techniques are employed to remove ABs from WW to prevent hazardous impacts on human health and the environment. Figure 5 gives an idea about the published research on the removal of ABs and highlights the most studied ABs. Based on this review, it can be concluded that tetracyclines and quinolones are the most studied AB classes. According to AB data (refer to Fig. 1), penicillin and tetracycline are the most consumed ABs in the selected countries from both the developed and developing world. In developing countries, penicillin and quinolones are found in the environment more frequently18,21,41,44,119,120. The published research proves that sorption techniques for the removal of ABs are promising and economical as they use various waste materials to produce low-cost adsorbents. These processes are capable of treating low as well as high concentrations of AB-rich WW. The sludge can be treated by incineration. The NaOH-activated carbon made from macadamia nut shell121 is reported to be the most efficient (100% removal of tetracycline) adsorbent. The minerals like montmorillonite, rectorite, and illite also have excellent AB removal efficiency (95%)100. It can be concluded from exiting literature that there is a need to check the feasibility of the developed biochar treatment process and to have an emphasis on the pilot-scale model.

This review gives a holistic idea about the AB consumption, sources of AB contaminants, contaminant cycle, effects, and removal techniques. It also highlights the following key issue: there is a significant literature gap existing in the understanding of fate, persistence, and transportation of by-products after the treatment of ABs. An advanced protocol must be developed to conduct the ecotoxicity test and evaluate its adverse effects on the environment. There is a need for developing predictive models to forecast the fate, persistence, pathways, and behavior of ABs in the aqueous systems.

## Discussion

In the past two decades, the occurrence and fate of AB contaminants in the environment have drawn exclusive attention of the scientific community and policymakers. The residues of ABs found in the different environmental matrices lie in a wide range of concentrations (ng L−1 to mg L−1). The concentration in the aquatic environment is increasing day by day since the application of ABs is extensive in humans as well as in veterinary sectors. ABs are enduring and have high resistance to decomposition, but are deposited in different environmental matrices. AB residues are harmful to humans, aquatic life, and the ecosystem even at low levels (i.e., at very low concentration). Due to this, several techniques for AB removal come into the picture to resolve the problem of AB contamination effectively.