A comprehensive review on BPA degradation by heterogeneous Fenton-like processes Open Access

Bisphenol A (BPA) is extensively utilized in synthesizing polycarbonates and epoxy resins, usually applied as liners in food contact materials, electronic components, constructing materials, phones, dental fillings, thermal receipts, and water bottles (Česen et al. 2018). It possesses endocrine-disrupting characteristics. It is a rapidly surging pollutant (Dietrich et al. 2017). Endocrine-disrupting chemicals (EDCs) are xenobiotic compounds that affect hormonal activity and cause myriads of adverse outcomes in the endocrine system and consequently affect the health of organisms, their progeny, or (sub) populations (Gmurek et al. 2017). These compounds have been identified in the aquatic environment, including groundwater, surface water, runoff, wastewater, landfill leachate, and drinking water. The occurrence of EDCs in aquatic systems is a growing concern, even at very low concentrations (Karthikeyan et al. 2019; Zhou et al. 2020). As proposed by the European Commission, EDCs can be removed from water by applying physical, chemical, or biological methods. Advanced oxidation processes (AOPs) can also be applied to mineralize EDCs. As EDCs pose harmful effects on living organisms and conventional technologies are not efficient in fully degrading several EDCs, advanced treatment is necessary to mitigate the estrogenic activity that exists at very low concentrations (Ying et al. 2008; Liu et al. 2009).

In this era of development, a shift from conventional water treatment processes is observed with the development of new treatment technologies. The findings of recent researches show that advanced oxidation processes (AOPs) are efficient and environmentally friendly for the degradation of BPA. Different types of AOPs include ozonation (Wang et al. 2021b), photocatalytic degradation using UV light irradiation (Hunge et al. 2021), sunlight-induced degradation (Daskalaki et al. 2011), and Fenton process (Huang et al. 2012a). Among the AOPs, the heterogeneous Fenton process, also known as catalytic wet peroxide oxidation (CWPO), is documented as a low-cost process as it requires simple equipment and mild working conditions (Thomas et al. 2021). Another report also indicates the cost-effectiveness of the heterogeneous Fenton process (Papic et al. 2009). In the heterogeneous Fenton process, hydrogen peroxide is used as an oxidizing agent, and a suitable catalyst is employed to stimulate its partial decomposition to hydroxyl radicals (•OH). The hydroxyl radicals (•OH) generated are strong oxidizing species that can efficiently degrade or mineralize the organic pollutants to simpler products (Ribeiro et al. 2016). The basic difference between homogeneous and heterogeneous Fenton processes is that no support is used in the former. In the latter, the iron catalyst is supported, which helps in the easy recovery of the catalyst. In the recent decade, other metals apart from iron have been used (Mahamallik & Pal 2016; Biswas & Pal 2020), termed as Fenton-like processes. Apart from H₂O₂, other chemicals such as peroxymonosulfate have also been practised to generate powerful radicals for oxidation (Kakavandi et al. 2022). The use of stable oxidants such as calcium peroxide has also been documented in some reports (Ali et al. 2020a, 2020b, 2021).

The Fenton process is an age-old process dealing with the generation of powerful hydroxyl radicals. However, in the recent past, enormous research has been carried out globally regarding the advancement of the Fenton-like process for the degradation of emerging contaminants like BPA. Several techniques have emerged, like the use of metal-free catalyst, the introduction of peroxymonosulfate, less use of traditional iron salt, and many more. So, it is of utmost importance to have a look at the recent trends in the field and highlight the existing challenges still associated with the applicability of the Fenton-like process for real wastewater in large-scale treatment containing BPA, and besides that, understanding the mechanisms of different kinds of Fenton-like techniques is also a matter of interest. Hence, the overall aim of this review article is to have a look at the latest development of the Fenton-like process in the field of BPA degradation, highlighting the different mechanisms of powerful radicals generation by using other novel catalysts, to have a comparative discussion regarding the suitability of different catalysts under varying operating environmental conditions, and the practice of optimization techniques to get the maximum benefits from the developed catalyst. Although the whole review mainly describes different heterogeneous Fenton-type catalysts, however, there is mention of some reported homogeneous systems which is relevant to the topic.

In this review work, reports that appeared up to 2022 have been discussed. The present work offers a summarized report on the use of solid catalysts to degrade BPA using the Fenton reaction. Thus after a few introductory and general sections describing the sources and effects of BPA, removal techniques of BPA and the main application of the Fenton chemistry with reasons why catalytic Fenton processes are important and various types of solid catalysts used in the degradation of BPA in the Fenton process will be discussed in detail. Specific highlights on the conditions in terms of pH, temperature, and H₂O₂ concentration under which the catalysts have been applied will be addressed. This review aims to examine (i) the adverse effects of BPA on living organisms, and (ii) its removal from the environment using the heterogeneous Fenton and photo-assisted Fenton method. The use of a wide range of heterogeneous catalysts for BPA degradation has been discussed here in detail. Moreover, the degradation mechanisms of BPA using the heterogeneous Fenton process have been demonstrated to describe the reactivity of catalysts towards BPA degradation.

The occurrence of BPA in the aqueous environment and its toxic effect has instilled an interest among researchers to find out some efficient treatment methods for the degradation of BPA.

The presence of BPA in natural waters has gained the attention of the public and the regulatory bodies because of its endocrine disrupting activity. Different levels of toxicity of BPA have been described. The LC₅₀ values for shrimp and minnow are 1.1 and 4.7 mg/L respectively, and the EC₅₀ value for water flea is 10 mg/L (Yamamoto et al. 2001). The poor biodegradability of BPA in nature leads to the contamination of surface water and groundwater.

BPA is released into the environment through numerous sources. It is usually discharged into the environment by the diffusive entry of BPA-containing materials and sewage water treatment plants (SWTPs). Several investigations indicate that the release of BPA occurs following different pathways such as the manufacturing process, from wastewater due to incomplete treatment or through physicochemical and biological treatment unit processes, from landfill leachates, and also due to leaching from discarded BPA-based products (Bhatnagar & Anastopoulos 2017; Dietrich et al. 2017).

In paper mills, BPA is used as a reactive agent for temperature-sensitive paper with color-developing layers during paper making. During recycling, recycled paper products such as toilet paper contribute BPA to wastewater. The sludge generated from paper recycling is used as fertilizers in agricultural activities, which finally lead to BPA contamination in groundwater (Mohapatra et al. 2010). BPA enters groundwater through leaching from waste plastics buried in a landfill, where the hydrolytic or leaching process releases BPA into the leachate (Yamamoto et al. 2001). BPA also contaminates food through different sources such as food processing, bottling, and packaging material, from where it can migrate into food by contact with resins, plastics, lacquers, paints from containers, and gaskets (Wetherill et al. 2007).

The global production of BPA by the end of 2015 was over 5 metric tons per year, indicating its widespread consumption. BPA can mimic the structure and function of hormones like estrogen, androgen, and hydrocarbon receptors, which may affect gene expression (Doshi et al. 2011).

The presence of BPA in different environmental mediums and food affects living organisms through the consumption of food and drinking water. Some metabolites of BPA are toxic to aquatic organisms. The usage of BPA is vast; therefore, human exposure to BPA is unavoidable, and as a result it imposes health hazards through estrogenic activity. BPA mainly induces estrogenic effects, but it also affects the metabolic, thyroid hormone, and androgen systems. It is also carcinogenic, genotoxic, and it causes a reduction in sperm count (Salian et al. 2009; Ohore & Zhang 2019). Chronic diseases such as prostate and breast cancer, type 2 diabetes, obesity, and impaired brain development occur due to early exposure to BPA (Cimmino et al. 2020). Therefore, it is essential to develop efficient methods for BPA removal from wastewater and freshwater systems to curb the negative health impacts.

Water pollution, a global environmental issue, has received considerable attention over the years for its remediation. Various methods involving physical, chemical, and biological techniques have been used for wastewater treatment but due to certain shortcomings of these methods, such as expensive catalysts, difficult regeneration and recovery, large occupied area, and poor treatment efficiency, these methods have limited applications (Li et al. 2021). Advanced oxidation process (AOP) is one such technology that is effective in degrading recalcitrant organic pollutants into inorganic compounds such as salts, CO₂, and water through the generation of reactive oxygen species (ROS) (Andreozzi et al. 1999; Thomas et al. 2021). Among several advanced oxidation technologies, classical Fenton oxidation (hydrogen peroxide and iron catalyst) has been widely used to oxidize recalcitrant organic compounds (Bokare & Choi 2014). Classical Fenton was discovered by Henry J. Fenton in 1894, who utilized H₂O₂ and soluble iron (Fe²⁺) to generate hydroxyl radicals at an acidic pH condition. However, this process holds a major disadvantage of iron sludge generation which needs further treatment and proper disposal (Umar et al. 2010). The complexities and costs involved in the classical Fenton reaction have restricted its use in wastewater treatment (Raji & Mirbagheri 2020). The major pitfalls associated with the classical Fenton process lead to the development of heterogeneous Fenton and Fenton-like processes. The modified process is cheaper, efficient, and has a wide pH working range of 3–7 (Nidheesh 2015; Devi et al. 2016). In heterogeneous Fenton reaction, iron oxides and iron oxide-supported porous materials are used as Fenton catalysts with H₂O₂, whereas in heterogeneous Fenton-like oxidation process (HFOP), a solid-phase catalyst such as Fe³⁺ or other transition metal ions are used with different oxidants for generation of ROS (Garrido-Ramírez et al. 2010; Rezaei & Vione 2018; Hussain et al. 2021). HFOP is an effective and promising method for the destruction of a wide range of contaminants present in different environmental matrices (Garrido-Ramírez et al. 2010; Babuponnusami & Muthukumar 2014). In HFOP, oxidants like hydrogen peroxide (H₂O₂), peroxymonosulfate (PMS, HSO₅⁻), and peroxydisulfate (PDS, S₂O₈²⁻) can be activated by solid catalysts to produce highly active hydroxyl radical (^•OH) (•OH, E⁰=2.8 V) and sulfate radicals (SO₄^•–) (E⁰=2.5–3.1 V) (Zhou et al. 2019). PMS and PDS have recently gained attention due to the strong redox potential (E⁰ (HSO₅⁻/SO₄²⁻)=1.75 V vs. NHE and E⁰ (S₂O₈²⁻/SO₄²⁻)=1.96 V vs. NHE) of the active radical species and their broad application pH range (Ahn & Yun 2019; Giannakis et al. 2021). SO₄^•– generated by activation of PMS and PDS is more powerful, selective, and effective than ^•OH. In addition, SO₄^•– are quite stable, environmentally friendly, cost-effective, and can be easily stored and transported. They have higher selectivity and longer half-life than •OH (Zhou et al. 2019). So, sulfate radical is expected to show similar or better capacity in the degradation of pollutants (Wang & Wang 2018). In this review, we have discussed the application of different oxidants in HFOP for BPA degradation.

A wide variety of Fenton-type catalysts have been documented in the literature regarding the mineralization of BPA from aqueous media. The variety of catalysts is large, so is their synthesis procedure. Li et al. (2022) used impregnation followed by a thermal method for the synthesis of tannic acid modified electro Fenton catalyst. Goncalves et al. (2020) synthesized humic acid-coated magnetic particles via the co-precipitation technique. Ding et al. (2020) prepared micro nanostructured CoS via a solvothermal method. Du et al. (2016) synthesized sulphur-modified iron oxide Fenton-like catalyst via a novel template-free method. Chu et al. (2021a) prepared a series of boron-doped graphene-wrapped FeS₂ nanocomposites (FeS₂@BrGO). In a nutshell, 0.1 g Fe₃O₄@BrGO was mixed with 1 g sulphur, and the resulting mixture was subjected to calcination in a tube furnace at 450 °C for 4 h. Various research groups reported several techniques for catalyst preparation. Hu et al. (2018) reported the preparation of Co₃O₄-Bi₂O₃ catalyst for BPA oxidation. Briefly, at first equal amounts of Bi(NO₃)₃.5H₂O and Co(NO₃)₂.6H₂O were added to HNO₃ and the mixture was subjected to continuous stirring till all the ingredients were thoroughly mixed. After that, the solution was transferred dropwise to NaOH and then the mixture was moved to a microwave reactor, and kept at 75 °C for 30 min. When the reaction mixture was cooled to room temperature, a black precipitate was obtained, which after drying was subjected to calcination at 400 °C for 2 h. However, among various preparation methods, in most cases drying in a tube furnace or calcination was the common final step.

Guo et al. (2020) prepared iron nanoparticles by using grape seed extract. Grape seed extract acted as the reducing agent for the conversion of Fe(II) to Fe(0), which was used as catalyst in the Fenton-like BPA degradation study.

In the earth's crust, the fourth most common element by mass and with compounds in several oxidation states is iron (Frey & Reed 2012). Iron compounds are usually considered nontoxic and eco-friendly (Xu et al. 2012; Pouran et al. 2014; Nidheesh 2015). Iron oxides and hydroxides are abundant minerals in the soil environment and are readily produced (Garrido-Ramírez et al. 2010). Among the available iron oxides, magnetite (Fe₃O₄), hematite (αFe₂O₃), maghemite (γFe₂O₃), and goethite (αFeOOH) are extensively utilized in heterogeneous catalysis (Pouran et al. 2014).

Iron oxide-based catalysts have exhibited superiority in heterogeneous catalysis because of their negligible toxicity, high catalytic activity, low cost, and easy recovery techniques. These catalysts can be synthesized by solvothermal, hydrothermal, microemulsion, thermal decomposition, and co-precipitation methods (Garrido-Ramírez et al. 2010; Pouran et al. 2014; Nidheesh 2015). The physical properties of the materials depend on the synthesis methods (Carvalho & Carvalho 2017). Some of the supported Fe₃O₄ catalysts are discussed in the following sub-sections.

Magnetite (Fe₃O₄) with mixed valences is the only pure common form of iron oxide. Its application in the heterogeneous Fenton process as a catalyst is extensive due to its nontoxic nature, low cost, large availability, and easy recyclability (Matta et al. 2007; He et al. 2014; Munoz et al. 2015). According to the Haber–Weiss mechanism, structural ferrous ions have a significant role in the initiation of ferrous ions (Costa et al. 2006). It is commonly selected as a catalyst mainly for two specific reasons. Firstly, the Fe²⁺ sites present on Fe₃O₄ react with H₂O₂ to generate •OH, and secondly, Fe₃O₄ has low solubility in an aqueous medium (Hou et al. 2016). In addition to its redox properties, magnetite has magnetic properties, which makes it easily separable after the degradation process (Sun et al. 1998; Pereira et al. 2012). Magnetite has been effectively used as a heterogeneous Fenton catalyst for BPA degradation. Nowadays, immobilization of Fe₃O₄ on organic or inorganic support has gained attention as a heterogeneous catalyst (Bao et al. 2015; Ma et al. 2015). Lai et al. (2018) carried out the degradation of BPA with vanadium-titanium magnetite (VTM). VTM was activated by PMS, and it effectively degraded BPA having the concentration 50 mg/L with ≥90% degradation efficiency using fixed VTM dose of 12 g/L and PMS concentration 4 mM.

Cleveland et al. (2014) used magnetite on a multi-walled carbon nanotube (Fe₃O₄/MWCNT) as a heterogeneous catalyst in the presence of H₂O₂, which resulted in the rapid and efficient elimination of BPA from an aqueous solution. The carbon nanotube provides a large surface area, good dispersion of iron oxides, and a high reaction rate, but its small size creates a problem in their removal after treatment. So, the amendment of multi-walled carbon nanotube with Fe₃O₄ exhibited high oxidation efficiency for the BPA degradation and imparted magnetic property, which assisted in effortless catalyst separation after treatment. The adsorption of BPA on the surface of Fe₃O₄/MWCNT was negligible, and it was primarily due to the structure of Fe₃O₄/MWCNT in which MWCNT was fully covered by Fe₃O₄, and the hydrophobic interaction of BPA with the surface of Fe₃O₄/MWCNT was interrupted by the charged surface of Fe₃O₄. The BPA removal efficiency at [H₂O₂]:[BPA] (mol H₂O₂/mol BPA) ratio of 4 was 97% in 2 h with a catalyst dose in the range of 0.5–1 g/L. Moreover, the [H₂O₂]:[BPA] ratio of 4 was cost-effective and generated nontoxic intermediates/products. The recycling of Fe₃O₄/MWCNT catalyst was effective for up to five consecutive cycles, which resulted in 90% removal of BPA. This further proves the high stability of Fe₃O₄/MWCNT. The authors found that the by-products and the intermediates formed in the BPA degradation were biologically nontoxic.

Zhang et al. (2019a) prepared Fe₃O₄@ β-CD/rGO by a one-pot solvothermal method, and applied the material for bisphenol A (BPA) removal. Fe₃O₄@ β-CD/rGO is a multi-component catalyst where β-cyclodextrin (β-CD) and reduced graphene oxide (rGO) are co-modified with Fe₃O₄ using a one-pot solvothermal method. rGO is a carbonaceous material with a large surface area, excellent chemical stability, and high conductivity. The active sites on the surface of rGO facilitate the catalytic reaction, whereas its hydrophobicity promotes strong interactions with organic pollutants in the aqueous medium. In this document, the authors reported that the higher surface area of graphene provided active sites for catalytic reaction of contaminants, and β-CD enhanced the solubility of the contaminants captured around the catalyst, which further encapsulates them in its hydrophobic cavity to allow exposure and close proximity to the generated hydroxyl radicals. The performances of the as-prepared catalysts viz., Fe₃O₄@ β-CD/rGO, Fe₃O₄@β-CD, Fe₃O₄/rGO, and Fe₃O₄ were compared. The catalytic degradation of BPA followed pseudo-first-order-kinetics, and the rate constant (k_obs) values were 0.02173 min⁻¹ for Fe₃O₄@β-CD, 0.09735 min⁻¹ for Fe₃O₄/rGO, 0.01666 min⁻¹ for Fe₃O₄ and 0.15733 min⁻¹ for Fe₃O₄@β-CD/rGO composite. The composite Fe₃O₄@β-CD/rGO was found as the best among all catalysts considered, and the enhanced catalytic activity was attributed to the development of a ternary complex (iron-β-CD-contaminant). The analysis of the BPA degradation products was carried out, and the intermediates were identified by LC-MS. The effects of pH, a dose of catalyst, H₂O₂ concentration, and NH₂OH intake on the removal efficiency were determined. Iron leaching from the first set of reaction with fresh catalyst was 0.0523% of total iron. The catalyst Fe₃O₄@β-CD/rGO composite retained the catalytic activity for up to five cycles, and the BPA degradation was found as 78.2±2.4%.

Hua et al. (2014) observed the effect of heterogeneous Fenton catalyst, Fe₃O₄/GO, on BPA degradation. The catalyst was prepared in situ by allowing the growth of Fe₃O₄ nanoparticles on the graphene oxide (GO) surface. GO is a carbon-based material and has gained attention recently due to its large specific surface area, electronic properties, high mechanical strength, and chemical stability. The surface of GO is hydrophobic, which allows a strong interaction with organic compounds. The experimental studies on various parameters with Fe₃O₄/GO showed an improved effect of GO on the adsorption of BPA molecules which expedited the process of degradation. The degradation at a concentration level of 20 mg/L of BPA with 1.0 g/L of Fe₃O₄/GO and 20 mM of H₂O₂ was ∼ 90% in 6 h and at pH 6.0. It was also observed that with an increase in H₂O₂ concentration from 5.0 to 30.0 mM, the BPA degradation increased from 82.7 to 88.4%, and at H₂O₂ concentration >30 mM, the efficiency of degradation decreased due to the •OH scavenging effect by H₂O_2. The authors concluded that Fe₃O₄/GO served as a potential catalyst for the degradation of BPA (Hua et al. 2014). Under optimum conditions, the ratio of catalyst Fe₃O₄/GO and H₂O₂ was 1.47.

Ferrihydrite (Fh) is a naturally occurring hydrous ferric oxyhydroxide mineral that is present in the earth's crust in large quantities. It is an iron-based nano mineral having a size in the range of 2−6 nm and a large specific surface area. Oxidized multi-walled carbon nanotubes (CNTs) are carbon-based nanomaterials with negatively charged groups on their surface. They have the potential to enhance the heterogeneous Fenton reaction by accelerating the redox cycling of Fe(III)/Fe(II) by decreasing the redox potential of Fe(III)/Fe(II), which facilitates the decomposition of H₂O₂. Zhu et al. (2020) synthesized a novel and efficient heterogeneous catalyst CNTs/Fh by a facile stirring method; 1 %CNTs/Fh, 3 %CNTs/Fh, 5 %CNTs/Fh, and 10 %CNTs/Fh catalysts were prepared by adding appropriate amounts of oxidized CNTs (0.01, 0.03, 0.05, or 0.1 g) and Fh powder (1 g) to 100 mL ultra-pure water. The specific surface area (SSA) of pure Fh, CNTs, 3 %CNTs /Fh, 5% CNTs/Fh, and 10 %CNTs/Fh were 273, 139, 387, 298.4 and 295.4 m²/g respectively. From the zeta potential results, it was found that CNTs and Fh have strong electrostatic interaction below pH 6.5. The degradation study of BPA was carried out using Fh, CNTs, 1, 3, 5 and 10 %CNTs/Fh catalysts. The degradation efficiency was compared, and it was observed that 3 %CNTs/Fh exhibited the highest catalytic activity. The high catalytic activity was due to the increase in SSA with an increase in CNTs content up to 3%, but with further increase in CNTs, SSA decreased. So, all the studies were carried out with 3 %CNTs/Fh. The BPA degradation achieved was 96% in 30 min, and the reaction followed a pseudo-first-order kinetic model with an apparent rate constant k_app=0.0811 min⁻¹ which is 7.1 times higher than that obtained using pure Fh with k_app=0.0114 min⁻¹. The TOC removal achieved during BPA degradation using 3 %CNTs/Fh was 79.1%. The degradation efficiency after four cycles of operation using 3 %CNTs/Fh was 99.6%. The reactive oxygen species like hydroxyl radicals (•OH) and superoxide radicals (O₂^·−) were responsible for the reaction. The study revealed that carbon-based materials and iron (hydr)oxides have scope in wastewater treatment and soil remediation (Zhu et al. 2020).

Iron oxides are attractive because of their abundance and low cost. The thermal and chemical stability, along with easy synthesis protocols and separation procedures, make them indispensable catalysts for Fenton-like reactions (Pinto et al. 2012; Pouran et al. 2014). Du et al. (2016) synthesized mesoporous iron oxide (M-Fe) and mesoporous sulfur-modified iron oxide (MS-Fe) for BPA degradation. MS-Fe was prepared by sulfur doping through the precipitation calcination process. MS-Fe with abundant porous channels was efficient in generating •OH radicals through H₂O₂ activation. MS-Fe developed a strong acidic microenvironment, and sulphur had an electron-mediating effect which promoted the fast activation of H₂O₂. It was observed that BPA degradation efficiency of 4% was achieved in 30 min of reaction time with M-Fe, whereas complete degradation of 0.2 mM BPA was achieved with MS-Fe within 30 min of reaction at a pH range of 3.0–9.0. The BET surface area of 151.6 m²/g and pore volume of 0.343 cm³/g of MS-Fe was much larger compared to that of non-porous sulfur-modified iron oxides-Fe (S-Fe), which showed the surface area and pore volume of 47.1 and 0.176 cm³/g, respectively. The kinetics of BPA degradation followed pseudo-first-order under various reaction conditions, and the main reactive species involved in the BPA degradation were hydroxyl radicals. The authors concluded that MS-Fe was an efficient catalyst that showed fast BPA degradation with a low concentration of H₂O₂ and catalyst dose. This fact widens its scope to be applied as an efficient catalyst for environmental remediation.

Guo et al. (2020) studied the BPA degradation using green iron nanoparticles (GS-Fe-NP) synthesized from grape seed extracts. The effects of GS-Fe-NPs (0.30 g/L) alone and H₂O₂ (1.0 M) alone after 150 min of reaction indicate BPA removal with an efficiency of 6 and 3.1%, respectively. On the other hand, similar experimental conditions in the presence of GS-Fe-NPs/H₂O₂ proved to be much better as they involved adsorption coupled with Fenton-like oxidation. It was observed that at an initial pH of 6.9, H₂O₂ concentration of 1.0 M, and catalyst dose of 0.30 g/L, the efficiency of BPA degradation was 96.4%. The degradation kinetics followed pseudo-first-order model suggesting that BPA degradation is dominated by the redox process. The TOC removal efficiency was 60% in 150 min. The studies carried out at different initial pH conditions indicated that at an initial pH of 3.0, complete BPA degradation could be achieved, and at pH 6.9, 93% BPA degradation was observed in 120 min. However, with a further increase in initial pH to 9.0 and 11.0, the BPA degradation of 92 and 83%, respectively, were obtained in 150 min. The increase in degradation at lower pH was attributed to the ^·OH generation, which was produced by the decomposition of H₂O₂ in acidic conditions. At higher pH, the decreased degradation efficiency was due to the formation of weaker oxidants like ferryl ion (FeO²⁺). It was also established with quenching experiments that ^·OH radicals were the primary radicals that promoted BPA degradation. The efficiency of radical generation varied with initial pH. The activation energy (E_a) calculated was 128.8 kJ/mol (i.e. >29 kJ/mol), which indicated that degradation of BPA in the GS-Fe-NPs/H₂O₂ system was a surface-controlled reaction. The degradation pathway was understood in LC-MS analysis, and in this report, 10 different intermediates were identified.

Apart from iron-based catalysts, BPA degradation was also tested using copper-based catalysts. Pachamuthu et al. (2017) conducted the Fenton-type degradation of BPA by TUD 1 silica-supported CuO catalyst. The catalyst was synthesized by a hydrothermal method utilizing tetra ethylene glycol as the structure-directing agent. It was found that the copper loading had an influence on the catalytic activity. In the study, 2.5% wt of Cu/TUD-1 showed optimum catalytic activity of 90.4% BPA degradation. However, only 68 and 78% removal were obtained with 5.4 and 0.9 wt% of Cu/TUD-1. Under optimum conditions, the ratio of Cu and H₂O₂ was found as 1.58×10⁻³. Xu et al. (2018) prepared Cu-Al₂O₃-g-C₃N₄ and Cu-Al₂O₃-C dots and examined their catalytic performance for Fenton-type degradation of different organic contaminants, including BPA. It was quite interesting to observe that Cu ions were able to coordinate with the hydroxyl group of tri-s-triazine ring of g-C₃N₄, and the graphene conjugated π-domains of C dots resulted in the formation of an electron-rich Cu center and electron-deficient π-electron conjugated system. This actually strengthened dual reaction centers, and further experimental studies validated that the two-electron transfer process produced •OH in the presence of H₂O₂. The electron-rich-centers to H₂O₂ produced the first electron, and the second was produced from H₂O to the electron-deficient site resulting in the formation of more hydroxyl radicals and hence better utilization of H₂O₂.

Wang et al. (2017a) prepared novel Cu-Al₂O₃ membranes via electrospinning technique and applied the same for BPA degradation by heterogeneous Fenton process. The copper species remained distributed over the fibers in the form of Cu⁺ and Cu²⁺, which was accountable for Fenton-type degradation. Upon addition of H₂O₂ at a neutral pH, more than 87% of the initial BPA was found to get degraded in the presence of 1wt% Cu-Al₂O₃ catalyst membrane. The experiments were conducted with 10 mL of BPA solution (initial concentration=20 mg/L) in the presence of 10 mg of membrane catalyst and 12 mM H₂O₂. Experiments were carried out at pH 3, 5, and 7, and it was noticed that the degradation efficiency remained the same at all pH conditions. Since, in most cases, the pH of natural water is near neutral, pH 7 was maintained for the experimental purpose. Finally, the leaching of Cu and Al after the reaction was checked, and it was found as negligible. It proved that the catalyst was stable enough, and thus it was suitable for practical applications.

Xie et al. (2018) prepared layered-shaped fungal manganese oxides by Mn(II) oxidizing fungus, and later it was transformed into manganese oxides (MnO₂, Mn₂O₃, Mn₃O₄) having different morphologies by thermal treatment. The manganese oxides thus prepared were utilized as a catalyst for BPA degradation in the presence of PMS. The catalysts so formed were found to be highly efficient in BPA degradation. Furthermore, the catalyst formed (FMO 400) upon calcination at 400 °C was capable of 97% TOC removal within 30 min.

Cobalt has been well known for its catalytic activity due to the possession of variable valences. Some studies also offer insights into BPA degradation using a cobalt-based Fenton-like process. Li et al. (2018) reported BPA degradation via a heterogeneous Fenton-like process catalyzed by N-doped graphene-supported cobalt. From the experimental investigations and density functional theory (DFT) calculations, it was evident that the CoN₄ site containing a single Co atom was the active site for peroxymonosulfate (PMS) activation. The turnover frequency (TOF) for the optimized catalyst has been reported as 12.52 min⁻¹. From the radical scavenger experiments and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) trapped electron paramagnetic resonance (EPR), it was confirmed that the singlet oxygen were generated from the activation of PMS on CoN₄ and was primarily responsible for rapid BPA degradation. Wu et al. (2020a) carried out a detailed study on the application of amorphous CoS_x cages for the degradation of BPA along with other recalcitrant compounds such as tetracycline, ciprofloxacin, methyl orange, and rhodamine B. About 90% of BPA was degraded within 20 min. Ding et al. (2020) synthesized a micro-nano structured CoS catalyst via the solvothermal procedure and applied the same for BPA removal by PMS activation. The effect of sulphur on the catalytic activity was systematically examined, and it was noted that under the action of 0.05 g/L catalyst and 0.3 mM PMS, about 90% BPA of 20 mg/L concentration was removed. The kinetic data were fitted to the pseudo-first-order model, and the rate constant found was 0.37 min⁻¹.

Graphene nanosheet supported nano cobalt sulphide crystals (Co₃S₄@GN, CoS@GN) were prepared by Zhu et al. (2019) via facile ligand exchange route using a metal-organic framework (MOF) as self templates. In the next step, thermal annealing was applied to convert Co₃S₄ to CoS, which played a vital role in activating PMS to form sulfate radicals to degrade BPA via the Fenton-like technique. The degradation was quite fast, and complete degradation was possible within 8 min. The experimental data fitted well to the first-order kinetic model with a rate constant of 0.62 min⁻¹ which was much higher in comparison to most of the reported heterogeneous catalysts. To justify the catalytic activity of CoS@GN, the degradation experiments were carried out in the presence of only PMS, in the presence of only CoS@GN catalyst (without PMS), and in the presence of both. It was observed that only PMS was not able to degrade the BPA, and only 20.39% adsorptive removal of BPA took place in the presence of CoS@GN (but no PMS). However, in the presence of both CoS@GN and PMS, the oxidation of BPA was rapid and efficient.

Bezerra et al. (2021) synthesized heterogeneous iron-based catalysts supported on biopolymers such as alginate (Alg), carboxymethylcellulose (CMC), and xanthan gum (GX) and chitosan (Qui) and their mixtures for the degradation of BPA. Six catalysts, Alg-Fe, QuiGX-Fe, CMC-Fe, CMCGX-Fe, AlgCMC-Fe, and AlgGX-FE, were synthesized and characterized by SEM, FTIR, and FAAS. Among the above catalysts, the highest catalytic activity was observed in AlgCMC-Fe. However, a small amount of iron ion leaching was observed in the reaction medium. The kinetic studies with various parameters were carried out with AlgCMC-Fe, and it could effectively remove 100% of BPA, having a concentration of 2 mg/L in 2 h at a near neutral pH. BPA degradation analysis was performed with HPLC-DAD. The removal efficiency of the catalyst was effective up to three cycles, and the degradation kinetics followed pseudo-second-order model. The authors concluded that AlgCMC-Fe is a new biodegradable and nontoxic support based on a stable form of iron (Fe) with good recycling potential, thus making it a more attractive catalyst.

Chalcopyrite nanoparticles (CuFeS₂ NPs) were synthesized by a modified hydrothermal process for BPA degradation (Nie et al. 2019). The S²⁻ present in the catalyst surface promoted the Fe³⁺/Fe²⁺ and Cu²⁺/Cu⁺ cycles on the surface, and the synergism of Fe³⁺/Fe²⁺ and Cu²⁺/Cu⁺ played a major role in PMS activation. The degradation of 20 mg/L of BPA with 0.1 g/L of CuFeS₂ NPs and PMS concentration of 0.3 mM was 99.7%, and TOC mineralization of 75% was achieved in 20 min at initial pH 6.0. Considering the molecular weight of PMS as 113 g/mole, the ratio of catalyst and PMS is 2.94. From ESR and quenching experiments, it was confirmed that hydroxyl radical (•OH) and sulfate radicals (SO₄^•–) were generated in the CuFeS₂-PMS system. ESR experiments also confirmed that the SO₄^•– generated through PMS activation was converted to •OH at pH above 7. This transformation of SO₄^•– to •OH decreased the oxidizing ability of the CuFeS₂-PMS system because of the low redox potential and shorter half-life time of •OH. Kong et al. (2019) synthesized Cu₂FeSnS₄ (CFTS) nanoflowers by hydrothermal method for BPA degradation. Complete degradation of 0.1 mM of BPA with 1 g/L of Cu₂FeSnS₄ and PMS concentration of 5 mM was obtained. CFTS showed high stability and catalytic activity towards PS activation for BPA removal through a surface catalytic process. A tandem synergistic effect between Cu, Fe, and Sn in their quaternary chalcogenide systems was witnessed for PS activation. From EPR and radical quenching experiments, it was observed that •OH and SO₄^•– were responsible for BPA degradation.

Metal-free catalysts have been developed to reduce the cost and the impact of metallic catalysts on the environment. They are promising green catalysts for wastewater remediation. Lin & Zhang (2017) prepared sulfur-doped carbon nitride (CNS) by one-step heat treatment of trithiocyanuric acid. CNS is a metal-free compound which activates PMS for BPA degradation. The effects of operational parameters like CNS loading, PMS dosage, temperature, pH, and coexisting ions were examined. BPA of concentration 50 mg/L was completely eliminated at CNS loading of 500 mg/L and PMS dose of 300 mg/L. The visible light irradiation (150 W) on CNS-activated PMS exhibited higher BPA degradation than CN-activated PMS. The increased catalytic activity was due to the synergistic effect of sulfur and nitrogen co-doping, which stimulated the catalytic/photocatalytic activities of CNS. Ren et al. (2020) conducted BPA degradation in a real wastewater matrix using amorphous boron as metal-free catalysts for activating PMS. The degradation of 0.1 μM BPA was carried out with a 0.2 g/L dose of amorphous boron and 3.0 mM PMS at pH 3.0. The removal efficiency of BPA in the boron/PMS system was 92.2%. The EPR and quenching tests confirmed that hydroxyl and sulfate radicals were generated. The characterization studies revealed that the surface of amorphous boron was oxidized into B₂O₃ during the activation of PMS, which got dissolved in an acidic solution and exposed the fresh boron. Lyu et al. (2018) synthesized 4-phenoxyphenol (POP) functionalized reduced graphene oxide nanosheets (POP-rGO NSs) by a surface complexation and copolymerization process. The degradation of 10 mg/L of BPA was carried out with an optimized dosage of POP-rGO NSs (0.4 g L) and H₂O₂ (10 mM). BPA degradation efficiency of 75.7% was achieved in 120 min. POP-rGO NSs had dual reaction centers (e.g., electron-rich and electron-poor), which were formed on the C−O−C bridge. The electron-rich center around O led to the reduction of H₂O₂ to •OH, while the electron-poor center around C captured electrons from the adsorbed pollutants and diverted them to the electron-rich area via the C−O−C bridge. Thus POP-rGO NSs exhibited excellent Fenton-like activity, good stability, and fast mineralization in a wide pH range.

Optimization of the experimental parameters is one of the most important tasks in achieving the desired degradation of the target pollutant. In a Fenton-like oxidative degradation study, several parameters are often involved influencing the ultimate mineralization of the pollutant. Huang et al. (2017b) reported the application of magnetite and ethylenediamine-N,N'-disuccinic acid (EDDS) as the heterogeneous Fenton catalyst for degradation of BPA. The authors carried out experiments under varying operating conditions in order to optimize the solution pH, EDDS dose, H₂O₂ concentration, etc. The most favorable conditions for BPA degradation were found as pH at ∼6.3, H₂O₂ concentration of 0.5 mM, EDDS dose 0.1 mM, BPA concentration 0.5 mM, and magnetite dose 0.2 g/L. Under optimized conditions, 70% BPA degradation was achieved. Xiao et al. (2020) synthesized a novel hybrid catalyst composed of defective WO_3-x nanowires along with reduced graphene oxide for the BPA degradation. A detailed optimization study was carried out, and the removal efficiency under this optimal condition reached 83%. Huang et al. (2012b) carried out sono-Fenton degradation of bisphenol A using Fe₃O₄ nanoparticles as heterogeneous catalysts. Optimization studies were conducted by variation of solution pH, the concentration of H₂O₂, and the dose of the catalyst.

In the present day, modeling and optimization techniques are gaining attention for efficient design of experiments in order to maximize the yield. Therefore, rather than optimizing a single parameter for a particular experiment, multi parameters optimization is becoming more meaningful to the scientific community. In a heterogeneous Fenton system there are various parameters involved, and each of the parameters may have a complex inter-relationship which ultimately affects the overall efficiency, multi-parameter optimization gives more insight into the degradation phenomenon. Response Surface Methodology (RSM) and Artificial Neural Network (ANN) are frequently used for this purpose. Research articles are available in the literature dealing with such techniques for optimizing BPA degradation procedure via heterogeneous Fenton-like procedure. Yang et al. (2016) explored the RSM technique for optimizing BPA degradation using FeOCl/SiO₂ Fenton-like catalyst. Independent parameters considered for analysis were H₂O₂ concentration, pH of the solution, and temperature. Furthermore, the results were analyzed by using ANOVA. It was found from the analysis that the influence of temperature on the degradation process was not very significant. On the other hand, the relationship between H₂O₂ concentration and pH of the solution was crucial in determining the efficiency of the Fenton process. Wang et al. (2021a) also used the RSM technique for the optimization of BPA degradation using iron oxide perovskite montmorillonite composite as the catalyst. Based on the results obtained from RSM, it was seen that complete mineralization of BPA was possible when the dose of H₂O₂ was 249.57 μL, the pH of the solution was 3.89, and microwave power was 512.76 W. Chu et al. (2021b) utilized ANN for the optimization study. Three parameters, such as temperature, initial peroxydisulfate (PDS) concentration, and ultrasound power, were selected for the optimization study. It was found that the temperature had maximum influence (46.83%), followed by initial PDS concentration (40.54%), and then ultrasound power (12.63%).

A catalyst is considered to be successful and most economical if it can show promising efficiency in a real wastewater matrix also. So, it is also of great significance to investigate the capability of Fenton-like processes for the removal of BPA when it is present in real wastewater streams along with other pollutants. There are research articles available in the literature where BPA degradation has been successfully demonstrated via Fenton-like AOPs in real wastewater. Real effluents from industrial units are mostly complex and composed of various organic and inorganic pollutants. Moreover, it is often difficult to monitor the oxidation of micro-pollutant like BPA in such wastewater matrices (Ozyildiz et al. 2019). Li et al. (2022) pointed out that the pH of the wastewater matrix may become a serious issue for the efficiency of the electro-Fenton type process. It is true that the application of the heterogeneous Fenton-type process has overcome the hurdles faced by the homogeneous process to a great extent. However, still many electro-Fenton processes fail to perform efficiently at neutral pH.

Khandarkhaeva et al. (2019) reported an interesting phenomenon of BPA degradation by a Fenton-like process in a real wastewater matrix. Although the study has been carried out with homogeneous Fenton catalyst, the features pointed out by the authors may be helpful for heterogeneous Fenton catalysts also. The authors mentioned that the presence of dissolved organic matter (DOM) present in real wastewater systems often helps in enhanced degradation of BPA via the Fenton-like process for iron catalyst. It is so because DOM has the ability to form Fe³⁺ complexes at neutral and alkaline pH, which favors Fenton oxidation. On the other hand, the authors also mentioned that the presence of carbonates and bicarbonates hinders the iron catalyst for the Fenton process. Chu et al. (2021a) also mentioned that the presence of bicarbonates in the real wastewater matrix reduces Fenton catalyst during BPA degradation. The authors tested FeS₂ nanoparticles for electro-Fenton degradation of BPA in different types of real wastewater matrices. It was found that due to the high concentration of bicarbonates in tap water and river water, degradation efficiency was reduced to 60%. On the other hand, in real wastewater systems where bicarbonates were not present or were present in relatively low quantity, enhanced BPA degradation was observed.

Goncalves et al. (2020) explored humic acid-coated magnetic particles as efficient photo Fenton catalysts for BPA degradation purposes. The material performed well in a real water matrix for degradation of contaminants of emerging concern such as carbamazepine, ibuprofen, 5-tolylbenzotriazole, and BPA. Simulated sunlight facilitated the reaction.

In order to get full insight into the catalytic phenomenon, understanding the mechanism behind the radical generation is of prior importance. Basically heterogeneous Fenton process may be of metal-based conventional heterogeneous Fenton type catalysis, electro Fenton type reactions, photo Fenton type reactions, sono Fenton type reactions, and others.

This involves the in-situ generation of Fe²⁺ ions, therefore increasing the Fe²⁺ dissolved ion concentration. Electro Fenton process often includes both homogeneous and heterogeneous reactions. The homogeneous reaction occurs with the dissolved Fe²⁺ species, and the heterogeneous reaction occurs with iron oxide deposited on the cathode surface.

Different published articles described the electro heterogeneous mechanism for BPA degradation. Zhang et al. (2020a), in their recent work, exhibited a heterogeneous electro Fenton process through the novel catalytic particle electrode. Catalytic particle electrode helps in regeneration of Fe²⁺ ions for the continuation of Fenton-like degradation. Chu et al. (2021a) reported the BPA oxidation by boron-doped graphene shell wrapped FeS₂ nanoparticles for BPA oxidation by electro Fenton-like technique. Although degradation mechanism was presented as a combination of homogeneous and heterogeneous Fenton processes, the heterogeneous process was the dominating one. The main degradation was performed by the •OH present on the surface of the catalyst.

The photo Fenton process describes the principle of generation of more hydroxyl radicals from H₂O₂ by ferrous ions in the presence of UV light. However, as we are moving towards more sustainable development, in most of the research works of the current era, visible light is preferred in comparison to UV light. Recently researchers explored various semi-conductor materials which produced photo-generated electrons helping in the regeneration of Fe²⁺ ions for efficiently performing Fenton type degradation.

In recent times, various research groups have achieved success in Fenton-like degradation by employing ultrasound irradiation. When ultrasound irradiation is used, cavitation due to the microbubbles are collapsed, creating an extreme localized condition of pressure (∼500 bar) and temperature (∼5000 K). This condition has been proved to generate more reactive hydroxyl radicals and reduce the mass transfer barrier to a great extent. Thus the reaction proceeds under facilitated conditions.

In many studies, toxicity analysis was often carried out on a test microorganism. In the study of BPA degradation by UVC-based advanced oxidation process (in homogeneous system), the toxicity levels of the intermediate degradation products were determined by applying to Artemia salina (Sanchez-Montes et al. 2020). In various combinations (viz. S₂O₈²⁻ and NaOCl), the mortality rate decreased after 4 h. However, as per the theoretical expectation, the toxicity should be eliminated within 2 h. So, the experimental observations led to the conclusion that not only the parent compound but also the degraded products possess toxicity that persisted beyond 2 h. Further studies were carried out on various aquatic animals in order to get an insight into the acute and chronic toxicity of degradation products.

Shekoohiyan et al. (2020) carried out the bioassay test on Daphnia Magna species. The effect of the treated BPA effluent (initial concentration of BPA 20 mg/L) was compared with the untreated one. After catalytic degradation, the effluent showed less mortality than the untreated one. After 48 h, on being exposed to the treated effluent (initial concentration of BPA: 20 mg/L) mortality rate was 30%, while in the untreated one, it was nearly 100%.

Wang et al. (2015) identified the intermediate compounds produced during BPA degradation by means of HPLC and GC-MS analysis. Five compounds such as 4-isopropenyl phenol, hydroquinone, 4-hydroxy acetophenone, benzoquinone, and phenol were detected. These aromatic compounds were supposed to get oxidized to produce aliphatic compounds such as lactic, oxalic, and formic acid, which consequently got degraded to CO₂ and H₂O. The LD₅₀ values found for the compounds hydroquinone, 4-hydroxy acetophenone, benzoquinone, and phenol were 245, 1500, 25, and 270 mg/kg, respectively, which indicated that all of them are more toxic than BPA (LD₅₀: 2,400 mg/kg). However, on an investigation of the relative quantity of each compound, it was found that 4-isopropenyl phenol and phenol generated were further transformed into hydroquinone, whereas 4-hydroxy acetophenone and phenol were decomposed within 2 h, and the concentration of hydroquinone and benzoquinone got maximized within 10 min and then got diminished. Hence, the quality of the effluent produced after degradation was devoid of any harmful compounds, although these compounds were formed during the course of the reaction.

Regeneration and recycling study of the catalyst deals with the economics of the developed system. Li et al. (2015) reused the CoAlMn composite catalyst for BPA degradation. The concentration of BPA was 10 mg/L, catalyst dose of 0.02 g/L, and oxone dose of 0.15 g/L. For the evaluation of recyclability of the CoAlMn catalyst, it was subjected to reuse both in acidic and neutral pH for three consecutive cycles. It was noticed that during the first two cycles, the removal efficiency was almost constant. However, in the third cycle, it was reduced significantly. The reason behind this deterioration of the catalytic property might be due to the adsorption of the intermediate products and also the leaching of the metal oxides leading to the restructuring of the catalyst surface.

Wang et al. (2015) investigated the stability and recyclability of iron-copper heterogeneous Fenton catalysts. The leaching of iron during the course of the reaction was determined by the 1,10-phenanthroline method, and it was found that the concentration of the leached iron after the BPA degradation was much lower than that prescribed by standards of the European Union. Moreover, the magnetization curves predicted that the material could be easily separated by a magnet and could be dispersed again for reuse.

After the first run of BPA degradation, Xie et al. (2018) reused the manganese oxide catalyst for the next cycle, and it was observed that the performance efficiency reduced to 40% from 100% in the first cycle. To investigate the reason behind the deactivation of the catalyst, the characterization studies such as XRD, N₂ physisorption, XPS, and FTIR analyses were performed on the catalyst both before and after the reaction. From the XPS analysis, it was revealed that the carbon content on the catalyst surface got increased after the reaction while manganese content got reduced significantly. It implied that the organic matter from the intermediate degradation products covered the catalyst surface resulting in a loss of efficiency. To overcome this problem, the samples were subjected to heat treatment at 400 °C for 2 h, and after that, it was noted that the catalytic efficiency regained up to 100%.

Lin & Zhang (2017) observed that sulfur-doped carbon nitride (CNS) was able to activate PMS for more than five cycles and the regeneration efficiency was >90% for multiple cycles. The reused CNS was further characterized using XRD and SEM. The characterization studies showed a similar crystalline structure and morphology as of pristine CNS, which indicated the effectiveness of CNS in activating PMS for BPA degradation.

It is observed that the heterogeneous catalysts where metal leaching is minimal show good reusability. Moreover, the catalysts where adsorption sites get covered or poisoned, show less efficiency in recycling tests. Hence synthesis of a stable catalyst is essential for achieving good reusability.

In order to know the functional groups present in the catalyst and to have an insight into the catalytic degradation of BPA, FTIR analysis was often performed. In the FTIR spectrum studied by Zhang et al. (2019a), a characteristic band of Fe₃O₄ at 580 cm⁻¹ was observed. The appearance of a band at 1574 cm⁻¹ was observed due to stretching vibration of C–C on the introduction of rGO, and the peaks at 1051 and 3415 cm⁻¹ due to C–O–C stretching and O-H stretching indicated the successful introduction of β-CD into the composite. In the FTIR spectrum of mesoporous sulfur-modified iron oxide (MS-Fe), as reported by Du et al. (2016), new IR bands were observed at 1358, 1315, 1194, 1092 and 819 cm⁻¹ compared to mesoporous iron oxide (M-Fe). The new bands at 1358 and 1315 cm⁻¹ were attributed to the stretching vibration of the S–O group, and the bands at 1194 and 1092 cm⁻¹ were designated to surface-bounded SO₃²⁻/SO₂, which were generated through heat-decomposition of thiosulfate. A peak formed at 819 cm⁻¹ was assigned to sulphite. Guo et al. (2020) reported that the FTIR spectra of GS-Fe-NPs before reaction showed strong bands at 3403 and 1613 cm⁻¹, which corresponded to O–H stretching vibrations and C–C aromatic ring stretching vibration, respectively. A band at 1524 cm⁻¹ was attributed to the in-plane bending vibrations of –OH in phenols. Peaks at around 1300 and 1200 cm⁻¹ corresponding to the stretching vibration of the single bond between C–O and a peak around 1063 cm⁻¹ were due to C–O–C stretching absorption. FTIR spectrum of GS-Fe-NPs after BPA degradation showed peaks at 3420, 2927, 1670, 1402 and 1037 cm⁻¹. A weak peak at 2342 cm⁻¹ was attributed to carboxyl, and a shift in the band from 557 to 597 cm⁻¹ was ascribed to Fe–O stretching vibration of Fe₃O₄ and Fe₂O₃ confirming the formation of iron oxide nanoparticles.

Lin et al. (2019) carried out the FTIR analysis of Mn_0.6Zn_0.4Fe₂O₄ nanoparticles and observed a broad band at 3398 cm⁻¹ due to -OH stretching. Besides that, due to the existence of a large number of residual hydroxyl groups, the peaks were obtained at 1367 cm⁻¹. Moreover, bands were also observed at 1074 and 1637 cm⁻¹ due to C-O stretching and H₂O deformation vibration, respectively. The peak found at 569 cm⁻¹ was assigned to (Zn and Mn)-O band for intrinsic lattice vibration of tetrahedral coordination in the spinel structure.

Electron microscopic analysis is one of the most widely practised techniques for morphology analysis of the catalyst. Cleveland et al. (2014) carried out the TEM and SEM analysis, which indicated that MWCNTs were attached to Fe₃O₄ (100–150 nm). Fe₃O₄ had an octahedron crystal structure which signified a well-dispersed mixture of Fe₃O₄ and MWCNT with little agglomeration. TEM analysis conducted by Zhang et al. (2019a) showed that Fe₃O₄, Fe₃O₄/rGO, Fe₃O₄@β-CD, and Fe₃O₄@β-CD/rGO catalysts had spherical shapes with diameters in the range of 250–300 nm. Among the catalysts, Fe₃O₄/rGO and Fe₃O₄@β-CD/rGO had a coating of a graphene sheet on Fe₃O₄ nanospheres, and the size of the composite was 50 nm less than Fe₃O₄ particles. A similar decrease in the diameter of the composite with cyclodextrin was observed, and it was interpreted that β-CD and GO effectively reduced the surface energy of the magnetic nanoparticles to improve the dispersion of the sample. The TEM images of Fe₃O₄ nanoparticles on GO were analyzed by Hua et al. (2014). It was noticed that Fe₃O₄ nanoparticles had a diameter of ∼6 nm, and they were homogeneously dispersed on the GO surface. The SEM images of raw CNTs carried out by Zhu et al. (2020) displayed an integrated tubular structure with a diameter of 20–40 nm and a length of 200–500 nm, whereas oxidized CNTs showed a decreased length of nanotubes with the surface of the nanotubes eroded indicating that CNTs were oxidized by HNO₃. SEM image of Ferrihydrite (Fh) exhibited aggregations of nanoparticles. CNTs and Fh, when combined, presented fluffier particles with uniform distribution of these two particles due to their strong interactions. The SEM images of 3 %CNTs/Fh showed that around CNTs, Fh nanoparticles were wrapped, and some small particles of the size of 5–10 nm were noticed on the tubes indicating well dispersion of Fh on CNTS due to the formation of Fe–O–C bonds between CNTs and Fh. From the SEM carried out by Du et al. (2016), the images of mesoporous sulfur-modified iron oxide (MS-Fe) showed cubic shapes with abundant pores and cracks. These cracks and pores were formed during high-temperature calcination, which induced rapid decomposition of oxalate to carbon dioxide and water. The TEM images also confirmed the presence of mesoporous structure, and the internal structure of MS-Fe composite also exhibited particles in nano-size aggregates with mesoporous structures. Guo et al. (2020) carried out the SEM analysis of green iron nanoparticles, which showed a quasi-spherical shape with an average diameter of 50–100 nm. The polyphenols and the active groups in the extracts were responsible for the formation of nanoparticles. They covered the surface of green iron nanoparticles (GS-Fe-NPs) and acted as a capping agent to inhibit the aggregation and oxidation of particles. Pachamuthu et al. (2017) carried out the morphological analysis of mesoporous 2.5 wt% Cu/TUD-1 catalyst for BPA degradation. The SEM analysis revealed the presence of micron-sized irregular silica particles. The TEM analysis, on the other hand, indicated the presence of a wormhole-like network. From the FESEM images of Cu-doped AlPO₄ Fenton-like catalyst, it was observed that AlPO₄ particles were made-up of agglomerated rectangular platelets, and with the increased Cu content, shortening and widening nature of the catalyst was noticed, which clearly indicated that the crystal growth was controlled by Cu doping (Zhang et al. 2017).

From the SEM images of Co₃O₄, Bi₂O₃, and Co₃O₄-Bi₂O₃ catalyst it was clear that the structure of Co₃O₄ was the irregular blocky type with many nanoparticles on the surface (Hu et al. 2018). The shape of Bi₂O₃ was rod-like, and the surface of Co₃O₄-Bi₂O₃ was full of wrinkles.

Wang et al. (2017a) reported that the prepared Cu-Al₂O₃ catalyst was flexible enough, and it could be bent many times without having a crack on its surface. Moreover, SEM images revealed that the membranes were made of uniformly and randomly oriented nanofibers. There were several pores also on the surface, which might be advantageous for the permeability of the membrane. After being calcined at 600 °C, the average diameter of the nanofibers decreased to some extent with the increase in Cu concentration. For example, the average pore diameter was found to be 390, 360, 340, and 320 nm corresponding to materials having 1, 3, 5, and 7 wt% of Cu-Al₂O₃.

In order to ascertain the oxidation state and crystallinity of the involved metal or metal oxide catalysts, the XRD analysis is often studied. Cleveland et al. (2014) carried out the XRD analysis of MWCNT, commercial Fe₃O₄, and Fe₃O₄/MWCNT. In the study, Fe₃O₄/MWCNT indicated the presence of Fe₃O₄ with peaks of 2θ values at about 18.3, 30.1, 35.5, 43.1, 53.5 and 57.0°. Zhang et al. (2019a) studied the XRD pattern of Fe₃O₄, which indicated six characteristic peaks with face-centered cubic (fcc) crystal. The XRD spectrum of four products, Fe₃O₄, Fe₃O₄/rGO, Fe₃O₄@β-CD, and Fe₃O₄@β-CD/rGO, exhibited similar crystalline peaks indicating that the crystallographic texture of Fe₃O₄ was not altered after the introduction of β-CD and GO. Zhu et al. (2020) observed a similar XRD pattern with two distinct reflections at 26.0 and 43.8°, corresponding to the diffraction planes of CNTs as observed in raw multi-walled CNTs and oxidized multi-walled CNTs. Ferrihydrite (Fh) indicated two broad reflections at 35.1 and 63.3°. The XRD pattern of CNTs and Fh on combining had no change except for the appearance of the characteristic reflection of CNTs. XRD pattern of M-Fe studied by Du et al. (2016) showed weak and dull diffraction peaks, which indicated low crystallinity, whereas MS-Fe composite after calcination showed unidentified 2θ peaks at 17.2, 22.6 and 37.5°, which might be attributed to sulfur-doped iron oxide. Guo et al. (2020) observed that the XRD pattern of GS-Fe-NPs had no distinctive peaks, which indicated the amorphous nature of GS-Fe-NPs. An ingredient in the grape seed extract was identified with a wide hump shoulder peak at around 21°, and it was considered to play a major role in the dispersion of GS-Fe-NPs. A weak peak around 2θ=45° was observed, which indicated Fe⁰ in green iron nanoparticles and a few peaks at 2θ around 28, 35 and 61° related to Fe₃O₄, Fe₂O₃, and FeOOH, respectively, got strengthened after reaction with BPA. From the XRD pattern of the composite heterogeneous Fenton catalyst CuFeO₂ microparticles, Zhang et al. (2014) concluded that the catalyst was highly crystalline in nature, as indicated by the presence of sharp XRD peaks. According to the JCPDS card number 75-2146, space group, R-3M, it was concluded that only a single-phase rhombohedral structure existed in the composite. Jung et al. (2019) carried out the XRD analysis of two types of prepared MnO₂ catalysts. The catalyst, prepared by keeping the time duration of the hydrothermal process at 6 h, showed peaks at 2θ=12.1, 24.3, 36.9, and 65.9°, which clearly indicated the presence of δ-MnO₂. On the other hand, XRD spectra of the sample prepared by keeping the time duration of the hydrothermal process at 12 h showed peaks at 12.7, 18.2, 28.7, 37.5, 42.1, 49.8, 56.4, 60.1 and 65.6°, which indicated the presence of α-MnO₂.

In order to ascertain the crystalline phase present in the membrane catalyst Cu-Al₂O₃, Wang et al. (2017a) carried out the XRD analysis of the membrane calcined at 600 °C. No sharp peaks but only humps were visible in the XRD spectrum, which revealed that the crystallinity was absent in the Cu species, and it might be amorphous in nature. With the increase in calcination temperature, the peaks of crystalline ɤ-alumina were visible, but those of copper were absent.

In the study of BPA degradation by micro-nano structured CoS catalyst (Ding et al. 2020), the changes in the oxidation states of Co and S in the fresh and used catalyst were analysed by XPS analysis. In the spectrum of Co 2p of fresh CoS catalyst, the three peaks that appeared at 777.6, 780.05 and 782.6 eV were assigned to Co²⁺, Co³⁺, and Co²⁺-OH, respectively.

In order to have an insight into the chemical valence changes that occurred in the BFO-MO catalyst, an XPS analysis of BFO-MO was carried out by Li et al. (2020) both before and after the degradation reaction. From the XPS of the fresh catalyst, the peaks of Fe2p_3/2 and Fe2p_1/2 were observed at 710.5, 724.6, 718.6, and 732.3 eV, respectively. The peaks of Fe 2p_3/2 and Fe 2p_1/2 were split into two peaks, 710.3 and 723.6 eV, which denoted the presence of Fe(II), while the peaks at 711.4 and 724.6 eV were referred to Fe(III) respectively.

From the XPS of the FMO-400 catalyst, the Mn 2p spectra of both before and after BPA degradation were analysed, and it was observed that the Mn(IV) to Mn(II)/Mn(III) ratio got reduced after degradation. A small quantity of Mn(IV) on the surface got reduced to Mn(II) during the reaction (Xie et al. 2018).

In order to get an idea regarding the specific surface area of the catalyst and the pore size distribution, BET analysis is often practised among researchers. Wang et al. (2015) carried out the BET analysis of mesoporous carbon-supported bimetallic iron-copper nanoparticles. A high specific surface area of 639 m²/g was obtained. On the other hand, the specific surface area of mesoporous carbon was 573 m²/g. This indicated that the incorporation of bimetallic nanoparticles increased the specific surface area of the composite. The reason behind this was that the incorporation of iron and copper nanoparticles prevented the shrinkage of mesoporous carbon during the carbonization process. Li et al. (2015) conducted a BET analysis of the CoMnAl-MMO catalyst. The specific area was found to be 64.17 m²/g, and the nitrogen adsorption-desorption isotherm showed similarity with type IV isotherm. It implied that the material was mesoporous in nature.

It was felt that there should be a comparative analysis among the different novel catalysts based on their physical properties. Such a discussion is presented in Table 3.

Fenton reaction undoubtedly is one of the most powerful techniques for decontamination of wastewater containing emerging pollutants like BPA. However, in the recent past, significant development has been noticed in the process optimization, material selection, and monitoring techniques for the determination of intermediate by-products formed during the course of the reaction. The present review describes the usage of various heterogeneous Fenton catalysts for BPA degradation and hence recommends the application of various unexplored materials for catalyst preparation. Utilization of the new age materials such as g-C₃N₄ (Xu et al. 2018) and MXenes (Liu et al. 2018b) for heterogeneous catalyst preparation is interesting. Apart from utilizing novel materials for catalyst preparation, the incorporation of innovative modifications in the existing systems can also produce new systems for catalytic action. Zhang et al. (2019b), in their review, mentioned the applicability of three-dimensional electrodes for the electro Fenton process. It was formed by packing some particles between a conventional two-dimensional system. There is a high probability of improvement in degradation efficiency in employing a three-dimensional system because of the possession of a large specific surface area, shortening of the distance between reactants and electrodes, high space utilization efficiency, etc.

Apart from discussing the advantages and latest developments in the Fenton like AOP for BPA degradation, the scope of the present review also includes some current challenges in this field and recommendations for possible solutions. One of the important challenges regarding the applicability of novel materials for Fenton-like degradation in the real wastewater scenario at a large scale is the hindrance to large-scale production of catalyst (Li et al. 2022). To make a Fenton catalyst suitable for large-scale application, only a techno-economic study is not sufficient to estimate the overall cost incurred. Additionally, the fund required for maintenance purpose needs to be assessed properly. The next hurdle may be the reduction in efficiency of iron-based catalyst for Fenton-like degradation in the presence of bicarbonate in real wastewater. Some promising catalysts have been reported as already stated in section 7, which show the potential to overcome this barrier.

Ribeiro & Nunes (2021), in their recent review article, highlighted some of the important perspectives which need serious attention of the scientific community. The authors emphasized the regeneration, reusability, and leaching of the catalyst. From a practical and economical point of view, the catalysts should have low leaching property and high reusability in order to transform the linear economy into a circular economy. To find out sustainable materials for preparing heterogeneous Fenton catalysts, a life cycle analysis should be conducted in detail. Additionally, the authors also pointed out that the real wastewater treatment with heterogeneous Fenton catalyst is still fewer in number. Rigorous experimental work needs to be performed in this field in order to understand the applicability of different heterogeneous catalysts to different types of real water matrices. As the presence of diversified coexisting substances may alter the catalytic efficiency, a study in real wastewater is of prior importance.

Besides developing new heterogeneous Fenton systems and their applications, monitoring degradation by-products also constitutes an important aspect of the degradation study. In many studies, while identifying intermediate by-products, it has been observed that there is a problem of signal overlapping. Mazivila et al. (2019), in their recent review, highlighted this problem and suggested various analytical techniques based on multivariate curve resolution alternating least squares, parallel factory analysis (PARAFAC) models etc. Aftab et al. (2018) also discussed the success of the implication of the PARAFAC model in analysing degradation products of landfill leachate and also identifying dissolved organic matter. Therefore, two and multi-way calibration techniques are highly recommended for product analysis. Moreover, leaching of original catalyst material after the degradation of BPA is also required to be investigated, which, however, remained unreported in various studies.

The authors are grateful to IIT Kharagpur for providing the research facility and Ministry of Education, Government of India, for financial support.

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

The authors declare there is no conflict.