Phthalate esters: occurrence, toxicity, bioremediation, and advanced oxidation processes

Contaminants of emerging concern (CECs) also known as emerging contaminants/pollutants are the miscellaneous group of compounds which are neither monitored routinely nor regulated properly. Emerging contaminants are significant nowadays because there is a recent increase in the presence of these contaminants, and it poses health risks to humans and animals (Sengupta et al. 2014). They include personal care products (i.e., detergents, soaps, cosmetics, nail paints, shampoos); plastic-derived compounds (i.e., phthalates, bisphenol); medicines (i.e., prescribed and non-prescribed); pesticides; and chemical products (i.e., preservatives, additives) which make its way into the aqueous system via wastewater treatment plants, sewage, urban land surface, and agricultural runoff, septic systems, etc. (Pironti et al. 2021). It will remain a hot topic of interest because new compounds are continuously evolving and are produced, let it be chemical pollutants (pharmaceuticals, phthalates, and bisphenols) and its by-products or biological pollutants (SARS-CoV-2 virus). Some of these CECs act as endocrine-disrupting factors (EDFs) which change the hormone level in organisms leading to reproductive disorders. There are other health issues related to CECs which include respiratory disorders, neurological disorders, and thyroid disorders (Pastorino & Ginebreda 2021).

The production of phthalate all over the world has grown from 2.7 to approximately 6 million tons per annum from 2007 to 2017 (Gao et al. 2018). Understanding the origin of phthalates in the environment as well as their distribution is important to evaluate the risk posed to the ecosystems. They are also used in personal care products and household plastics, which leads to higher levels of phthalates in urban wastewater plants.

Phthalates are intractable in conventional wastewater treatment, therefore making their way into the surrounding lakes and rivers where it gets accumulated in the environment and the system of fishes and other aquatic species, later entering the food web which leads to noxious effects on humans and other biotas (Kasonga et al. 2021). The conventional treatment methods like adsorption, and flocculation, biodegradation are not effective in the PAE remediation process as the issue arises for the disposal of the adsorbate, large amounts of sludge production, non-biodegradable compounds, etc. Therefore, it is important to understand the upcoming remediation methods used. Advanced oxidation processes have been proved to be an efficient and emerging method for the degradation of phthalates, they include a wide range of processes like photocatalysis, photolysis, Fenton processes, ozonation, UV-based methods, sonolysis, electrochemical methods, radical-based methods and coupled AOPs. Coupling together various AOPs like photo-Fenton processes, sono-Fenton, O₃/UV process, O₃/H₂O₂, and PMS/O₃ has also provided efficient degradation of the phthalate esters. AOPs have been suggested to be used in conjunction with secondary wastewater treatment methods.

The objective of the present review is to (i) understand the types of PAEs and their occurrence globally, (ii) presents a summary of the PAE toxicity and biodegradation study and (iii) document the advanced oxidation processes used for phthalate acid esters (PAEs) removal. This information will provide a basis for effective framing of regulations to limit the level of phthalates in the water and soil systems as well as an overview of the state-of-the-art methods evaluated recently for the degradation of phthalates.

VOS viewer, Cite space, and Hist cite are some common software used for bibliometric analysis. VOS Viewer (version 1.6.18) is an effective software tool for assembling and analyzing bibliometric networks from literature data. Network visualizations are convenient for the researchers to visualize and interpret. The network visualization of ‘co-occurring keywords’ plays an important role as it reveals the important area of the research field, more the frequency observed for a keyword indicates that more research is being carried out in that particular area. This software constructs maps between related data. The networks quantitatively analyze the title, keywords, and word frequency.

Total of 77 articles were extracted using the keywords (TITLE-ABS-KEY (advanced AND oxidation AND processes) OR TITLE-ABS-KEY (advanced AND oxidation) AND (phthalate AND esters) OR (phthalic AND acid AND esters)) AND PUB YEAR > 2011 AND (LIMIT-TO (DOCTYPE, ‘ar’)). The visualization density for ‘phthalate esters’ and ‘advanced oxidation processes’ can be observed in Figure 3(b). The main keywords are ‘phthalate’, ‘advanced oxidation processes’, ‘ultraviolet radiation’, ‘photodegradation’, ‘ozone’, ‘Fenton’, ‘catalysts’, etc. There is a total of six clusters observed in Figure 3(a) which concludes the main focus of the articles. Cluster 1 (dark blue) covers phthalate studies related to advanced oxidation processes with respect to UV, ozone, and activated carbon (AC). Cluster 2 (red) covers phthalate study with respect to AOPs using catalysts, PMS and iron compounds, and hydroxyl radicals. Cluster 3 (green) covers phthalate studies related to photocatalysis and the last cluster is a mixture of Fenton reaction related and toxicity related. With respect to AOPs and phthalate esters, major research can be observed under UV, ozone, oxidation–reduction. Further studies can be focused on sulfate radical-based, peroxide-based, and other hybrid AOPs.

The diesters of 1,2-benzenedicarboxylic acid are frequently introduced as phthalic acid esters (PAEs) or phthalates. Toxic PAEs present in the environment are dimethyl phthalate (DMP), diethyl phthalate (DEP), di-isobutyl phthalate (DIBP), dibutyl phthalate (DnBP), butyl benzyl phthalate (BBP), and di-(2-ethylhexyl) phthalate (DEHP). Ortho, meta, and para are the isomeric forms of phthalic acid based on the orientation of the carboxylic acid with one another. However, the ortho form of benzene dicarboxylic acid, which is developed when a particular alcohol reacts with phthalic acid to form the desired ester, is used mostly as the plasticizer and brings about the significant contribution of all phthalate esters manufactured globally. They are widely utilized for manufacturing plastic products, polyvinyl chloride and other polymers to increase longevity and flexibility (Gao & Wen 2016;Yang et al. 2018). Nearly 60 discrete phthalate esters and its by-products are used for numerous products, i.e., cosmetics, adhesives, elasticity, lubricants, plastic manufacturing, packaging material, insecticides, and additives in paints to increase the longevity and flexibility of the plastic product (Eichler et al. 2019). A list of toxic phthalates with its molecular weight, chemical structure, toxic effects, and applications in the industrial sector are mentioned in Table 1.

Table 1

Molecular weight, chemical structure, and application of some toxic phthalates

Phthalate esters .	Mol. weight and CAS .	Chemical structure .	Solubility in water (mg L−1) .	Toxic effects .	Applications .
Dimethyl phthalate (DMP)	194.2 and 131-11-3		2,014	Liver problems, reproductive difficulties	Air freshener, shampoo, insecticides, etc.
Diethyl phthalate (DEP)	222.2 and 84-66-2		287	Male reproductive issues, liver problems	Plasticizers, aerosol sprays, cosmetics fragrances, nail polish, etc.
Dibutyl phthalate (DnBP)	278.3 and 84-74-2		2.35	Teratogenic, testicular damage, carcinogenic	Cosmetics, anti-foaming agents, printing inks, wrapping materials.
Di-(2-ethylhexyl) phthalate (DEHP)	390.6 and 117-81-7		0.00	EDCs, Cytotoxicity; liver problems; elevated cancer risk.	Medical tubes, blood storage bags, gloves, furniture, diapers, children's dolls, packaging films, dialysis bags, and air tubes.
Butyl benzyl phthalate (BBP)	312.36 and 85-68-7		0.950	Cellular toxicity	Vinyl flooring, wrapping materials, carpet tiles, conveyor belts, automotive, adhesives, artificial leather.

Phthalate esters .	Mol. weight and CAS .	Chemical structure .	Solubility in water (mg L−1) .	Toxic effects .	Applications .
Dimethyl phthalate (DMP)	194.2 and 131-11-3		2,014	Liver problems, reproductive difficulties	Air freshener, shampoo, insecticides, etc.
Diethyl phthalate (DEP)	222.2 and 84-66-2		287	Male reproductive issues, liver problems	Plasticizers, aerosol sprays, cosmetics fragrances, nail polish, etc.
Dibutyl phthalate (DnBP)	278.3 and 84-74-2		2.35	Teratogenic, testicular damage, carcinogenic	Cosmetics, anti-foaming agents, printing inks, wrapping materials.
Di-(2-ethylhexyl) phthalate (DEHP)	390.6 and 117-81-7		0.00	EDCs, Cytotoxicity; liver problems; elevated cancer risk.	Medical tubes, blood storage bags, gloves, furniture, diapers, children's dolls, packaging films, dialysis bags, and air tubes.
Butyl benzyl phthalate (BBP)	312.36 and 85-68-7		0.950	Cellular toxicity	Vinyl flooring, wrapping materials, carpet tiles, conveyor belts, automotive, adhesives, artificial leather.

Mankidy et al. (2013); Zhang et al. (2015); Giuliani et al. (2020); Das et al. (2021).

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DMP, diethyl phthalate (DEP), and di-(n-butyl) phthalate (DnBP) have low-molecular-weight (LMW) PAEs. They are utilized in industrial solvents, personal care products and cosmetics, pharmaceutical products, insecticide, perfume solvents, adhesives, waxes, and inks (Koniecki et al. 2011). To evaporate the perfume fragrance very slowly, DMP and DEP are used, increasing the time span for the aroma of the scent, and in nail polish to develop chip-resistant properties which is provided by a small amount of DnBP. DEHP comes under high molecular weight (HMW) PAEs. About 80% of alkyl groups with long chains are the phthalate esters utilized as plasticizers in the PVC industry which upgrades the elasticity, flooring, wall-coverings, and basic handling properties. The binding of PAE with the polymeric matrix is not strong, which leads to the gradual release of this EDC into the environment (Net et al. 2015). Due to the manufacturing process, leaching, and weathering, there is constant release of this pollutant into the environment.

Phthalate esters have now become an omnipresent environmental contaminant because of their excessive application in various sectors and its easy release into the environment. The leading phthalate esters used in personal care products and cosmetics are DEP followed by DBP and DEHP. DEP exposure sources are fragrances and lotions followed by deodorants, whereas the main source of DnBP are nail polishes (Koniecki et al. 2011). Mostly the esters have low volatility, are colorless liquids and are poorly soluble in water. These esters are soluble in oils and organic solvents. PAEs have been used as elastomers and plasticizing agents in cellulosics. There is a desperate requirement for the observation of the sources, toxicity, and ecological risk of phthalate esters in various sources in different countries, particularly in urban areas with more phthalate-based companies (Zhang et al. 2015).

Phthalate esters are observed in surface water, fresh water, drinking water, air, sediments, and soil worldwide. Various environmental agencies like the United States Environmental Protection Agency (USEPA), Environment Canada, European Union, and China National Environmental Monitoring (CNEM) have classified DEHP as a priority pollutant (Zolfaghari et al. 2014). The European Union (2013) gave guidelines for the DEHP concentration and proposed limits in marine water to be 1.3 μg L⁻¹. DEHP (0.006 mg L⁻¹) is one among the 90 contaminants listed among the legal limits set by the US EPA (US EPA 2013). Phthalates are widely discharged from rubber and chemical factories. Phthalate esters enter the surroundings very easily as it is present in a leachable form inside the PVC and they are not covalently attached to the polymeric matrix. During the manufacturing process of PVC, phthalates are consistently released into the environment by weathering, evaporating. or leaching from the final products (Gao et al. 2018).

In the urban areas, due to industrialization and urbanization, phthalates and their esters in different forms are released in high concentrations from the plastic materials which result in higher phthalate concentration in urban areas as compared to rural areas. They are leached out from the plastics present in the environment and objects used in homes and house dust, and also in a particulate matter which is suspended in the surroundings and contaminates the environment. Continuous bioaccumulation studies in the environments are required to explain the stress generated by the elevated concentration of phthalate esters (Sun et al. 2013).

Various pathways lead to the release of phthalates into marine sources including wastewater treatment units at the industrial and municipal levels, agricultural and storm runoff, deposition in the atmosphere and transportation in the river resulting via the application of sewage sludge (Net et al. 2015). In a study performed in the Marne Aval wastewater treatment station, a total of six phthalates were detected and DEHP (9–44 μg L⁻¹) was found to be the major pollutant followed by DEP (1.6–25 μg L⁻¹) (Dargnat et al. 2009). The secondary treatment process in a wastewater treatment plant removes the maximum amount of DEHP. The average concentration of DEHP in wastewater gets reduced from 30.08 to 8.13 μg L⁻¹ (Takdastan et al. 2021). Other PAEs are intractable in conventional wastewater treatment, therefore making their way into the surrounding lakes and rivers where it gets accumulated in the environment and the system of fishes and other aquatic species, later entering the food web which leads to noxious effects on humans and other biotas (Kasonga et al. 2021).

The occurrence and estimated concentration of phthalate esters vary between 0.03 ng m⁻³ and 24.19 μg m⁻³ in air, 313 and 4,640 ng L⁻¹ in water, and 40 and 348 ng g⁻¹ in soil. The DMP concentration in different matrices was found to be N.D. –10.4 ng m⁻³ in atmospheric particulate matter, N.D. –31.7 μg L⁻¹ in fresh water, N.D. –316 μg kg⁻¹ in sediments (dry weight), and N.D. –43.27 μg L⁻¹ in landfills (Gao & Wen 2016). About 15 phthalate esters were detected with total concentration ranging from 0.386 to 3.184 μg L⁻¹ in lake samples (Zheng et al. 2014). The total occurrence of the six PAEs concentration analyzed from the Mediterranean Sea (Marseille Bay) ranged from 130 to 1,330 ng L⁻¹ (avg. 522 ng L⁻¹) (Paluselli et al. 2018). Seven PAEs and their monoesters were detected across four sampling areas in water from the Okavango delta, Northern Botswana (fresh water) with DEHP metabolites like MEHHP and MEOHP at high concentrations ranging from <1 to 29.3 ng L⁻¹ (Bartsch et al. 2019). Due to the rise in human activities, four dominant phthalates were detected in the lakes of Beijing, namely DIBP, DBP, BBP, and DEHP.

River sediments are excessively analyzed as an indicator for the environment to judge phthalate contamination since it bioaccumulates and is widely distributed. PAEs, particularly DEHP were detected in river sediments present worldwide with concentrations from 0.1 ng g⁻¹ to 100 μg g⁻¹ (Liu et al. 2013). Higher DEHP concentrations (1.56 μg L⁻¹) were found in tap water in wet months (Cui et al. 2022). For transporting phthalate esters with an approximate annual flux of 0.705–29.4 kg, storm water acts as an important contributor (Cao et al. 2022).

PAE concentrations are detected more in low river-flowing areas, specifically in reservoirs than the high river-flowing areas. Hence, the proposed and completed artificial human-constructed dams on the river will potentially aggravate organic compounds pollution by making changes in hydrology. Translocation of pollutants is inhibited particularly by low river flow, which results in exacerbating ecological risk in the river and enrichment in small areas. The results show the river flow rate effect on the distribution of PAEs and the effects of urbanization (Sun et al. 2013). Table 2 summarizes the concentration of different types of phthalate esters present in different regions.

Most of the PAEs are listed as emerging chemicals which disrupt the endocrine function and lead to various developmental and reproductive toxicities affecting reproductive health, physical development, immunotoxicity, neurotoxicity, teratogenicity, mutagenicity, and carcinogenicity for environmentally relevant exposures in various wildlife animals. DEHP is the majorly observed PAE utilized in PVC production as a plasticizer (Magdouli et al. 2013). Exposure tophthalate esters mainly occurs through inhalation, skin contacts causing bioaccumulation in human bodies. The interaction of this contaminant affects the various physiology of the human body causing teratogenicity, mutagenicity, and carcinogenicity even at minimum concentrations (Gao & Wen 2016). PAEs pose a great health hazard to living beings due to the fact that they are unavoidably exposed to phthalates from the consumption of surface water contamination and up-taking aquatic organisms (Sun et al. 2013). Figure 4 illustrates the disorders caused by phthalate esters in different organs of the human body like brain, lungs, heart, etc.

Potential pathways of exposure to phthalate esters in human beings are drinking water, contaminated foodstuffs, inhalation or dermal contact with the phthalate ester used in products like cosmetics and personal care products. Humans and other species are subjected to risk via inhaling, ingesting, or dermal exposure to the water which is contaminated. These pollutants are present at nanograms and micrograms per liter concentration in groundwater, surface water and also drinking water as a result of anthropogenic activities. The research on the concentration of phthalates presents in the beverages, surrounding air, foodstuffs we consume and other phthalate-containing things present in the lifestyle of humans should be monitored (Fawell & Ong 2012; Net et al. 2015; Tijani et al. 2015). PAEs are eliminated as conjugate monoesters in urine, and oxidative transformation via secondary metabolism takes place for DEHP before urinary excretion. In serum samples and milk, phthalate esters and their metabolite concentration were detected under the method detection limit which is 0.12–3.0 μg L⁻¹, whereas the concentration detected and analyzed in urine was found to be 0.1–1,000 μg L⁻¹. MEP, MBP, and MEHP were observed in samples collected from nails, because DEP is widely used in nail polishes (Wang et al. 2019a, 2019b, 2019c).

Exposure to phthalates at high concentration is reported to cause pregnancy complications and miscarriage in females (Tijani et al. 2015). Predominant phthalate metabolite which is MEP was detected in pregnant women's urine samples from various countries like the Netherlands, USA, Canada, Norway, and Spain. In Norway, PAEs were found to be 30 μg L⁻¹ and 246 μg g⁻¹ was detected in Spanish pregnant women. It has also been reported in the saliva samples, amniotic fluids, and breast milk of lactating mothers (Buckley et al. 2012; Arbuckle et al. 2014; Sabaredzovic et al. 2015; Valvi et al. 2015).

Elevated levels of around 2.09 μg L⁻¹ of DEHP concentration 1.75 μg L⁻¹ of DBP concentration and its metabolites have been detected in the semen samples of men from Germany and USA which leads to the partition in the seminal plasma (Buck Louis et al. 2018; Smarr et al. 2018). In research conducted to observe the daily intake of PAEs through ingestion, it was observed that children (21 months old) are more exposed than adults because of household dust and dirt, their hand-to-mouth licking habit as they often lick their toys where DEHP and its metabolites were found to be present (Weiss et al. 2018).

In Hong Kong, the mean PAEs concentration detected in 20 fish species from fresh water varied from 1.66 to 3.14 μg g⁻¹ in concentration by wet-weight (ww) and ranging from 1.57 to 7.10 μg g⁻¹ ww in marine fish, where DBP and DEHP were the predominant phthalate esters. These PAEs are considered to be accumulated during gastrointestinal digestion (Cheng et al. 2013). DEHP is entering water bodies on a daily basis which is a threat to the fishes and aquatic organisms’ health leading to health effects on the local surrounding people (Takdastan et al. 2021).

There are various methods like the adsorption process (using various matrix, biochar, and AC) and biological treatments (using a microorganism, and membrane bioreactors) are used for the remediation of phthalates from the water and wastewater system. Various wastewater treatment technologies when coupled together resulted in greater efficiency for the phthalate esters remediation than using the single treatment method. When the membrane bioreactor was coupled with anaerobic wastewater remediation, the degradation of PAEs was enhanced from 65–71% to 95–97% (Gao & Wen 2016). Becky Miriyam et al. (2022) explained in detail about the adsorption studies and membrane technology used for PAEs removal. Chitosan beads immobilized with lipase was a cost-effective method used for DMP and DEP removal which showed 100 and 93.86% degradation, respectively (Dulazi & Liu 2011). Phytoremediation is a method where the plants and their parts are being used for the remediation of organic pollutants via degradation, uptake, and enzymatic actions. Crude enzyme extracts from Oryza sativa L. used for the degradation of DBP indicated that the roots of the rice plant metabolized DBP much more effectively than the stems and leaves of the plant (Zhu et al. 2019).

Aerobic, strictly anaerobic or facultatively anaerobic microorganisms are found to degrade phthalate esters from the environment. Some unfamiliar enzymes or transesterification have been reported in most of the studies of longer alkyl residues getting converted to shorter ones (Boll et al. 2019). DMP is more biodegradable than others since this substrate has a maximum specific growth rate and has the highest values of overall reaction rate (Ahmadi et al. 2017). Glutamicibacter nicotianae ZM05 is a DBP-degrading exogenous bacteria and Cupriavidus metallidurans ZM16 is a non-DBP-degrading bacterium that is effective together for co-contamination degradation. These insights assist to explicate the mutual effects of fungal and bacterial communities for co-contamination degradation of DBP-Cd and bring forth new perceptions for the fabrication of degrading consortia for the breakdown of the pollutants. It is a considerable concern for the impact that the bioaccumulation potential and toxicity of PAEs have on the environment (Gao et al. 2018). Through co-culture experiment, it was proven that Pseudomonas aeruginosa ZM03 can elevate the degradation rate of Arthrobacter nicotianae ZM05 for DBP removal under stressed pH conditions (Wang et al. 2021a, 2021b). Table 3 lists the microorganism used for the biodegradation of phthalate esters and its concentration with the removal percentage and the time duration.

Yang et al. (2013) conducted DBP degradation in synthetic and actual wastewater with enriched mixed culture in a series of batch bioreactors where 200–1,000 mg L⁻¹ of initial DBP concentration was neutralized at 91–99% removal efficiency. High-performance liquid chromatography electrospray-ionization quadrupole time off light mass-spectrometry (HPLC-ESI-QTOF-MS) was used to determine some of the major PAEs metabolites when Bacillus mojavensis B1811 was used for the degradation (Zhang et al. 2018a, 2018b).

Hydrolysis of PAEs by esterase is the primary step which releases the free phthalate moiety and side chain alcohols in the degradation studies (Boll et al. 2019). Some by-products including mono-methyl phthalate, DMP, catechol and phthalic acid were detected in biodegradation of the specific phthalates exhibiting DAP and DEP which follows similar pathway for the degradation due to the detachment by side chain alkyl as per the MBBR effluent metabolite surveying results. During biodegradation of DEP, metabolites were observed, concluding that side chains of DEP get detached as the first step, and then mono-ethyl phthalate, mono-methyl phthalate and mono-ethyl phthalate were formed. De-methylation or de-esterification is the next step, which is followed by DMP, MMP, PA and catechol production, respectively (Ahmadi et al. 2015).

Various advanced oxidation processes like photocatalysis, electrochemical mineralization, sonolytic degradation, aqueous oxidation, Fenton oxidation, photochemical degradation, photocatalytic ozonation, and biochar facilitated degradation are applied to eliminate the phthalate esters based on the concept of adding or creating particles which are highly reactive and can oxidize molecules which are more stable (Julinová & Slavík 2012). These processes can be applied as a pre- or post-treatment method of the biological process of pollutant removal. Generation of •OH radicals play an important role in the process of pollutant degradation using AOP, which is carried out by photochemical, chemical, or sonochemical methods (Na et al. 2012a, 2012b).

The European Union Directive (12 August 2013) concludes Fenton-based processes to be the mostly used AOPs (31%), subsequently followed by heterogeneous photocatalysis (20%), whereas 18% of the research indicates a comparison of different advanced oxidation techniques for degradation studies in water for the contaminants present (Ribeiro et al. 2015). Becky Miriyam et al. (2022) explained electrochemical processes for phthalate degradation in detail. This review covers some important AOPs like photocatalysis, the Fenton process, ozonation, and sonolysis.

Photocatalysis is a favorable AOP technique utilized for the treatment/removal of contaminants by mineralizing and oxidizing it under favorable conditions. Various photocatalysts and photocatalysis-based works have been investigated in water bodies for phthalate ester degradation. The basic principle of this method is that the pollutant is treated under UV radiation with or without the chemicals. DEP photodegradation was observed with Graphene quantum dots/Mn-N–TiO₂/graphene-C₃N₄ composite when photocatalytic action leads to the production of H₂ molecules (Nie et al. 2018). The calcination method was used to prepare g-C₃N₄/Bi₂O₂CO₃ and g-C₃N₄/BiOCl which considerably increased the photocatalytic activity for DnBP degradation (Shan et al. 2016). Research shows that DEP degradation by persulfate activated by CuFe₂O₄/MWCNTs magnetic nanoparticles was effective at various factors like the concentration of PS, the amount of catalyst loaded and temperature. The catalyst exhibited more catalytic performance, stability and reusability, whereas an eligible amount of metal ions was leached out from the catalysts (Zhang et al. 2016). PANi/CNT/TiO₂ immobilized on glass plates were represented as an effective photocatalyst for DEP degradation (Hung et al. 2017).

Rapid DMP breakdown and •OH radical formation was observed using hollow glass microspheres covered with photocatalytic TiO₂ (HGM–TiO₂) (Jiang et al. 2013). Bi₂O₃–TiO₂ composite was effective under a visible light source for mixed pollutants of Pb (II) together with DBP degradation (You et al. 2018). In another study, Fe₃O₄@CuCr-LDH nanocomposites are used for the activation of PMS for the photocatalytic removal of DEP. This LDH-based S-scheme nano-heterojunction can be separated using magnets (Fazli et al. 2021). The degraded products of DMP were assessed using high-resolution (HR) Orbitrap mass-spectrometry (MS) assembled with ultra-high performance liquid chromatography (UHPLC) (Tan et al. 2020).

Jamil et al. (2017) performed the removal of total organic carbon (TOC) of DBP using Bi, Cu Co-doped SrTiO₃ with a visible metal halide lamp with a power of 82 μlm/W indicating increased removal. Vanadium pentoxide (V₂O₅)/molybdenum trioxide (MoO₃) composites include both doping and heterostructure which enhance the process and which is performed under a visible-light source at a wavelength greater than 420 nm using a high-pressure xenon lamp (500 W) for efficient removal of DMP (Chuai et al. 2015). Tungsten oxide (WO₃) deposited on the surface of TiO₂ showed a higher degradation rate than the bare TiO₂ photocatalyst under UV – LED and blue LED light sources (Ki et al. 2019). Nano-α-Fe₂O₃ presence with UV irradiation (300–400 nm) significantly increases the degradation rate of DEP (Shuai et al. 2018). The anatase phase, which is a crystalline size inhibitor, is produced when Ni is doped in TiO₂. The lower the crystalline size, the higher will be the disordering of the sample resulting in effective photocatalytic activity for the degradation of DEP (Singla et al. 2015).

Kaur et al. (2017) analyzed a DEP degradation study using transition metal-doped TiO₂ nanoparticles (Mn-, Ni-, and Co-doped TiO₂) under Hg lump (125 W) at 365 nm, where Ni-doped TiO₂ showed better results. Different ratios of reduced graphene oxide zinc oxide (rGO-ZnO) nanocomposites prepared using temperature refluxing methods showed efficient photodegradation of DEP (Kumar et al. 2021). Under UV radiation and magnetic separation, Fe₃O₄@TiO₂ core-shell nanoparticles which get operationalized using cyclodextrin plays an important role as a photocatalyst for DBP degradation and mineralization (Chalasani & Vasudevan 2013). Mono-methyl phthalate and phthalate are the two intermediates (aromatic) observed in the electrolysis of DMP which indicates the attack on the methyl esters group at the initial step in the oxidation process (De Souza et al. 2013). Organic layered double hydroxides (LHDs)/TiO₂ composites were effective in the adsorption and photodegradation of DMP and approximately 80% DMP was removed in 4 h (Huang et al. 2013). Table 4 shows different catalysts used for the photocatalytic process along with the light source and percentage of removal.

The UV-Fenton and Fenton processes reduced the level of DEHP and DBP, and also are considered as appropriate methods for treating concentrated solutions (Wang et al. 2016a, 2016b). According to Şolpan & Mehrnia (2018), phthalate degradation is affected by H₂O₂ concentration, and irradiation dose. When there is an increase in the amount of hydrogen peroxide, the degradation process is enhanced. DEP removal was enhanced when the Fe(III)/PMS process was combined with C-60 fullerenol (Zhou et al. 2020a, 2020b). According to Qi et al. 2020 combination of sono-Fenton with a photocatalytic process under visible light (Vis/P25) enhanced the mineralization process for DMP and DEP degradation, where better results were obtained for DMP degradation. Table 5 indicates the Fenton oxidation/Fenton-like processes with modifications for phthalate degradation with the removal efficiency.

The use of a gas diffusion electrode (GDE) fabricated from carbon material (CMK-3) was a much more effective electro-Fenton process for DMP degradation (Wang et al. 2013a, 2013b). A total of 93% of DiBP removal was observed from water using galvanic anode in electro-Fenton method due to the hydroxyl radical formation under various operating conditions, i.e., pH 5, space between the plate (5 cm), electrolyte (Na₂SO₄ with H₂O₂) (Yang et al. 2020a, 2020b). Homogeneous catalysts like mixtures of some transition metal ions that replaced the Fe²⁺ in the Fenton reagent achieved phthalate reduction (Ziembowicz et al. 2021). The PMS/MFe₂O₄ system and graphene-based CoFe₂O₄/PMS as sulfate and hydroxyl radical plays a crucial role in the phthalate degradation mechanism (Ren et al. 2015; Xu et al. 2015a, 2015b).

Chen et al. (2016) found that in the presence of clay minerals, the Fe content present in the clay structure leads to an effective Fenton reaction for the degradation of DEP with the formation of six by-products majorly mono-ethyl phthalate, followed by phthalate acid, hydroxyl DEP, and many more compounds. Ferric reduction reaction–electro-Fenton increased the H₂O₂ production in the gas diffusion device which leads to efficient DMP degradation (Liang et al. 2021a, 2021b). 99.1% of DMP degradation is observed using the electro-Fenton process with the generation of hydroxyl radicals in 8 min (Dolatabadi et al. 2021).

Ozone (O₃) is often used in water treatment because of its high oxidation activity with the contaminants (Gucheng et al. 2017). In the ozonation process, two different pathways are followed: (i) direct reaction with O₃ and (ii) indirect reactions with hydroxyl radicals. According to Mansouri et al. (2019), the AOPs which are effective for the oxidation and mineralization of a broad range of contaminants from wastewater are ozonation, ultraviolet radiation and H₂O₂ based. AOPs like O₃/H₂O₂ and O₃/AC are effective for DEP degradation when compared to the traditional methods (O₃ and UV alone) (Medellin-Castillo et al. 2013). On performing the three different ozonation processes (ozone alone, O₃/H₂O₂ and O₃/ZnO) for DEP mineralization, O₃/H₂O₂ showed better results (Wen et al. 2011). O₃ and UV/O₃ effectively reduced DEHP as compared to the conventional UV method, where pH plays an important role in the degradation process (Yang & Lin 2012). Whereas in a study for DMP degradation, UV/O₃ process performs better than the free O₃ process (Yang et al. 2020a, 2020b). AOPs such as UV/H₂O₂ were effective in alkaline conditions with natural organic matter (NOM) for DBP degradation (Wang et al. 2016a, 2016b). Table 6 depicts the list of ozone/UV-based AOPs for phthalate degradation showing the radicals generated and the removal efficiency of the process.

Table 6

Ultraviolet/ozone-based AOPs for phthalate degradation

Phthalate .	Catalyst/process .	Removal efficiency (%) .	Radicals generated .	References .
DMP	(Cu2O)0.5·CuO·Fe2O3 nanoparticles (CFO NPs)	100 (20 min)	•OH, •CH3 and radicals	Liu et al. (2019)
	Co-Mn-Mesoporous siliceous (MCM-41) catalyst	99.7 (15 min)	•OH radicals	Tang et al. (2017)
	Cerium-loaded SBA-15 (Ce/SBA-15)	88.7 (60 min)	•OH radicals	Yan et al. (2013)
DEP	Electro-peroxone with carbon – polytetrafluorethylene	99 (60 min)	•OH radicals	Hou et al. (2016)
DBP	Magnetic porous ferro spinel NiFe2O4	100 (60 min)	and O2 radicals	Ren et al. (2012)
	O3/UV process	100 (60 min)	H2O and CO2	Wang et al. (2013a, 2013b)
BBP	O3/UV process with scavenger	91 (15 min)	Tert butanol as radical scavenger	Lovato et al. (2014)
DEHP	V2O5/TiO2 – Ozone	58.7 (1 min)	•OH radicals	Tak et al. (2022)

Phthalate .	Catalyst/process .	Removal efficiency (%) .	Radicals generated .	References .
DMP	(Cu2O)0.5·CuO·Fe2O3 nanoparticles (CFO NPs)	100 (20 min)	•OH, •CH3 and radicals	Liu et al. (2019)
	Co-Mn-Mesoporous siliceous (MCM-41) catalyst	99.7 (15 min)	•OH radicals	Tang et al. (2017)
	Cerium-loaded SBA-15 (Ce/SBA-15)	88.7 (60 min)	•OH radicals	Yan et al. (2013)
DEP	Electro-peroxone with carbon – polytetrafluorethylene	99 (60 min)	•OH radicals	Hou et al. (2016)
DBP	Magnetic porous ferro spinel NiFe2O4	100 (60 min)	and O2 radicals	Ren et al. (2012)
	O3/UV process	100 (60 min)	H2O and CO2	Wang et al. (2013a, 2013b)
BBP	O3/UV process with scavenger	91 (15 min)	Tert butanol as radical scavenger	Lovato et al. (2014)
DEHP	V2O5/TiO2 – Ozone	58.7 (1 min)	•OH radicals	Tak et al. (2022)

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Catalytic ozonation for DBP degradation was analyzed using magnetic, stable graphene-MnFe₂O₄ hybrids. The synergistic function between the support matrix and the nanomaterial results in efficient degradation. Also, NiFe₂O₄ and Ag_0.1Ni_0.95Fe₂O₄ formulated using sol–gel process enhanced ozone decomposition during the DBP degradation process, and during the ozonation process it concludes that the removal is done by degradation and not adsorption. Catalytic ozonation of DMP by AC was performed on the basis of hydrodynamics, adsorption, mass transfer and reaction kinetics to enhance the catalytic ozonation reactors where gas-liquid continuous flow is present (Zheng et al. 2020; Yu et al. 2021; Ma et al. 2022a, 2022b, 2022c).

Ozone-based AOPs study like heterogeneous catalytic ozonation with Al₂O₃ and AC showed effective results for DEP removal, whereas in the presence of O₃/N-methyl hydroxylamine (N-HA) coupled with catalytic ozonation efficiently removed DMP with increasing the N-HA concentration (Gucheng et al. 2017; Mansouri et al. 2019). Heterogeneous photocatalysis coupled with ozonation is effective for the removal of DEHP from wastewater using Nx–TiO₂x nanoparticles. Ozonation if combined with other processes like photocatalytic reaction and sonolysis enhances the degradation rate of phthalates (Anandan et al. 2013). Novel Fe/Mn@γ − Al₂O₃/O₃ nano catalyst showed great efficiency in real water samples for DMP degradation (Liang et al. 2021a, 2021b).

UV/H₂O₂ method can be enhanced tenfold in the presence of ferric ions for DMP degradation, and the first degradation intermediate is dimethyl 4-hydroxy phthalate (DMP-4OH) (Du et al. 2015). For the degradation of DEP, UV/H₂O₂ is much more effective when compared with the traditional method (O₃, UV). UV/H₂O₂ and UV/TiO₂ were compared for DEP photodegradation, where UV/H₂O₂ proved to be highly effective for DEP removal (Medellin-Castillo et al. 2013; Song et al. 2015).

If ultrasound (US) is applied to an organic pollutant solution, a pyrolysis mechanism is generated due to cavitation inside the cavitation bubbles which leads to the production of •OH radicals (Na et al. 2012a, 2012b). High-frequency ultrasonic methods in combination with H₂O₂ can enhance the radical formation to improve the DMP degradation (Xu et al. 2013a, 2013b). Cavitation bubbles play an important role because in the ultrasonic method, the removal ability is similar to the amount of cavitation bubbles in the water due to the introduction of mechanical agitation (Nie et al. 2021). The combination of US with titanium dioxide (TiO₂) catalyst and UV irradiation known as sono-photocatalysis and sono-photolysis, respectively, of DEP, exhibited synergistic effects which were advantageous (Na et al. 2012a, 2012b). Phthalate degradation takes place via radical oxidation in the sonolysis process (Xu et al. 2015a, 2015b).

According to Souza et al. (2014), when UV-light irradiation and US are coupled to obtain improvement in conductive-diamond electrochemical oxidation (CDEO) for the treatment of DMP in synthetic wastewater. The collaboration of high-frequency US and catalyst-free UV radiation which is known as sono-photolysis is observed to perform effectively for DMP degradation. Also, effective sonochemical degradation of DMP was performed with high-frequency ultrasonic processes (HFSU) under various operating parameters. HSFU undergoes pseudo-first order kinetics after reacting with the *OH radicals, whereas the degradation rate is enhanced with the increase in the power density of US (Xu et al. 2013a, 2013b).

Na et al. (2012a, 2012b) analyzed the degradation efficiency with sonolytic, photolytic and sonophotolytic processes using US (283 kHz) and two different types of UV lamps (UVC-254 nm) and (VUV-185 nm + 254 nm), where H₂O₂ generated via UV irradiation and application of US to generate •OH radicals. Hence, enhanced DEP degradation indicates the major role of the US in sonophotolytic processes. Similarly, US treatment was given to manganese oxide catalyst for the activation of PMS efficiently, which resulted in the complete degradation of DMP (10 mg/L) in 90 min (Yi et al. 2022). In a study, sono-Fenton method combined with H₂O₂ and photocatalytic process under visible light was effective in degrading DMP and DEP (Qi et al. 2020). Use of TiO₂ at high-frequency enhanced the sonocatalytic degradation of DEP, whereas sonolysis was not a very effective method (Cho et al. 2012).

Several compounds are used to activate the peroxymonosulfate (PMS) which gives much more effective results as compared to bare PMS. Metal-organic frameworks like (Fe (II)-MOFs) altered with a particular molecular imprinting layer (Fe(II)-MOFs@MIP) can be used for DBP degradation as adsorption-catalytic degradation (Chi et al. 2021). The hydroxyl radicals increase the degradation step in advanced oxidation systems. Electron beam radiolysis can produce both •OH radicals and electrons which can be advantageous than the method where only •OH radicals are generated (Liu et al. 2012). Hydroxyl and sulfate radicals are initially responsible for DEP removal leading to the production of intermediary products (Zhou et al. 2020a, 2020b). Metal-free graphitic carbon nitride (g-C₃N₄) synergistically works with PMS for DMP degradation and mineralization which took place mainly due to the and •OH radical attack to the benzene ring and oxidation of the aliphatic chains (Xu et al. 2020).

Different combinations of AOPs like UV-activated PMS oxidation have shown effective results. In a study by Huang et al. (2017), activated PMS using UV was used for generating and •OH radicals which resulted in 90% DEHP degradation in 70 mins. Similarly, Lei et al. (2020) reported 98% removal of DEP in 10 min generating ⁠, •OH, ⁠, CO₃ and Cl radicals. The UV/PS process produced hydroxyl radicals (OH*) and sulfate radicals (⁠*) for effective DBP degradation which can be enhanced by increasing PS concentration. UV/PS is considered as an effective treatment method for phthalate contaminated water (Wang et al. 2018, 2019a, 2019b, 2019c). DEP degradation under alkaline conditions was effective for a UV/sulfite system with oxygen, since •OH accumulation increases the pH (Chu et al. 2021).

The catalytic oxidation of DMP is seen to be enhanced by α-Fe₂O₃–TiO₂ nanocomposite coupled with persulfate using non-thermal dielectric barrier discharge (NTP-DBD) plasma reactor (Ahmadi et al. 2020). PMS gets decomposed by a metal-free activator like amorphous boron to produce reactive oxygen species (ROS) for effective degradation of DEP (Ren et al. 2020). Oxidation of PMS was performed using a heterogeneous catalyst like Magnetic ferro spinel MFe₂O₄ (Metal = Co, Cu, Mn, and Zn) for DBP degradation, where MFe₂O₄ showed high PMS catalytic effect even after the seventh used (Ren et al. 2015). In a study, 99% DMP (20 mg L⁻¹) was removed in 90 min with PMS activated using manganese oxide nanomaterial (H₂⁻OMS-2) with abundant Bronsted-acid sites by means of ion-exchange of crypto melane-type MnO₂ (OMS-2) (Zhang et al. 2022a, 2022b).

Greenhouse gas, CO₂ was used to prepare a core-shell (Co@C) catalyst using molten salt electrolysis which activates the PMS to degrade DEP (Ma et al. 2022a, 2022b, 2022c). In a study, reduced Fe-bearing smectite clays are used for the activation of PMS which contributes to rapid DEP degradation in 30 s, where the and •OH radicals bind to the surface of the clay rather than mixing in the bulk solution (Chen et al. 2020). When 0.1 gL⁻¹ of LiCoPO₄ (LCP) was used as an activator for PMS (0.5 g L⁻¹) producing and •OH radicals, 98.6% OF DEP was removed in 6 min. Likewise, and •OH radicals are generated when nanoscale zero-valent tungsten (nZVW) is utilized for activating PMS for DMP degradation (Lin et al. 2017; Zhou et al. 2019). A (Co,Mn)₃O₄/PMS system, which is prepared using two metals, i.e., cobalt and manganese was considered more efficient for DEP removal than the single metal systems like Co₃O₄/PMS and Mn₅O₈/PMS. The synergistic effects between the Co and Mn oxides showed good utilization results for pollutant degradation by enhancing the PMS activity (Klu et al. 2022).

According to Lin et al. (2020) TOC estimation with the degradation of DEP showed that it can be mineralized into CO₂ and H₂O by the use of PMS activated using Co_0.59Fe_0.41P nanocubes prepared from nanoscale metal-organic frameworks, which removed 95% of DEP in 60 min. Similarly, MOF core-SiO₂ shell nanomaterial (Fe-MOF-74@SiO₂) prepared using a hydrothermal process, can be used for persulfate (PS) activation effective for DMP degradation. Also, (Fe-MOF-74) can be improved by a molecular imprinting technique which provides much more efficient results (Ding et al. 2021a, 2021b). In a study, cage-core PMS catalysts were synthesized using electrospinning–calcination of the cobalt–zeolitic imidazole framework (ZIF-67) crystals to remove 90.4% DMP in 60 mins (Pang et al. 2022). Using a ball milling process, a novel sulfur-doped activated carbon (SACx) particle was prepared to activate PMS which was effective in the degradation of DEP (Huang et al. 2021a, 2021b). Sulfate-radical-based AOPs are much more efficient than other AOPs methods as they can be combined with other AOPs present.

Biochar as a sorbent is applied for pollutant removal, whereas some research works have been performed with biochar merged with some AOPs which is an emerging topic of interest. According to Nidheesh et al. (2021), catalyst which is supported by biochar performs better than the unsupported catalysts for contaminant removal.

DEP degradation with partial mineralization was accounted for using UV radiation and solar lights suspended in biochar solution, due to the formation of quinone-like formation of bio carbon minerals (BCM-Q) and BCM-enclosed with persistent free radicals (BCM-PFRs) which is the principal factor affecting the production of ROS like •OH and . For the degradation of DEP, generation of ROS and hydroxyl radicals takes place from the biochar in the presence of oxygen. Catalase, oxide dismutase, and deferoxamine are utilized for the free-radical quenching studies to justify the mechanism. For phthalate degradation, •OH production plays an important role, where biochar carbon matrix-bound persistent free radicals’ activation and photo-Fenton steps are the route cause (Fang et al. 2015, 2017). Nitrogen doped over biochar improves H₂O₂ production due to O₂ reduction reaction leading to 86.8% of DMP degradation (Nidheesh et al. 2021).

Biochar-based study was performed for the first time, where carbon nanofiber (CCNF) prepared using cellulose (biochar) combined with cobalt ferrite (CoFe₂O₄) that forms a nanocomposite which is used to activate PMS to enhance the catalytic activity. It increases the transfer rate of the electrons present and blocking the agglomeration of the CoFe₂O₄ nanoparticles, which leads to the formation of radicals, i.e., and •OH where acts as the dominant active species (Gan et al. 2019). Complete DMP degradation within 60 min was observed with the PMS activated using biochar@CoFe-LDH composite (BC-LDH). The synergistic effect possessed by the biochar and CoFe-LDH results in higher catalytic activity and increased stability for the PMS activation (Ye et al. 2021). Xiong & Pei (2021) mentioned that persulfate activated using transition metal shows good results for PAEs degradation. Synergism of biochar and transition metal-PS can get over the disadvantages of a single approach and perform efficiently in the degradation process.

Biochar combined with PS, PDS, and PMS is a novel approach for PAE degradation. Red seaweed-derived biochar is used to activate sodium percarbonate (SPC) for DEHP degradation by undergoing carbo-catalysis by generating OH radicals (Hung et al. 2021). Magnetite nanoparticles (Fe₃O₄) – rice husk biochar (RHB) composite activated the sodium persulfate (Na₂S₂O₈, PS) to form for PAE degradation in marine environments (Dong et al. 2018). Biochar-based advanced oxidation process is an emerging concept for the contaminant's degradation. Magnetic biochar doped with Fe exhibited photocatalytic effects in the presence of solar radiation for effective remediation of DEP because of the formation of PFRs, •OH radicals, and radicals (Yi et al. 2021).

CECs/emerging pollutants are a major concern for the society in the present scenario because of the increase in anthropogenic activities. AOPs can generally achieve higher removal efficiencies of CECs from contaminated sites than the conventional treatment methods. Data gap exists for the occurrence of phthalate esters in India, which needs to be explored. Toxicology experiments related to phthalates need to be explored to understand the level of toxicity present in various subjects. Some future trends and prospects are mentioned in the following:

• Various by-products are formed as the oxidation of pollutants takes place, which could be potentially toxic. The extent of phthalate degradation and the resultant residue toxicity need to be evaluated.

• In the present scenario, AOPs can be coupled with the biological process which makes use of reactors and waste materials like biochar and hydrochar that may produce effective results. This step can be beneficial as the AOPs can remove the recalcitrant compounds that can impair the oxidation in biological processes. Whereas the biological processes can be advantageous as it overcomes the pre-oxidized water condition which is not possible with AOPs.

• Much less research related to hydrochar-based AOPs are explored for phthalate degradation. Less literature is available for biochar-based AOPs.

• The detection level of occurrence for phthalate esters was found to be in the decreasing order of magnitude in wastewater, surface water, groundwater and drinking water. It is important to identify efficient remediation techniques for appropriate concentrations.

• Less information is available on the industrial scale studies for phthalate removal, so further studies involving scaling up can be explored.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.