Applications of biochar in sulfate radical-based advanced oxidation processes for the removal of pharmaceuticals and personal care products Open Access

ACT

Acetaminophen

AOP

Advanced oxidation process

API

Active pharmaceutical ingredients

BC

Biochar

BTA

Benzotriazole

CBC

Corn stalk biochar

CBZ

Carbamazepine

CEX

Cephalexin

CIP

Ciprofloxacin

EDC

Endocrine disrupting chemicals

EfOM

Effluent organic matter

EP

Emerging pollutants

EPFR

Environmentally persistent free radical

FTIR

Fourier transform infrared spectroscopy

IBU

Ibuprofen

NOR

Norfloxacin

OFG

Oxygen containing functional group

OFX

Ofloxacin

PDS

Peroxydisulfate

PFR

Persistent free radical

PMS

Peroxymonosulfate

POP

Persistent organic pollutant

PPCP

Pharmaceuticals and personal care product

PS

Persulfate

ROS

Reactive oxygen species

SFG

Surface functional group

SMX

Sulfamethoxazole

SPS

Sodium persulfate

SR-AOP

Sulfate radical-based advanced oxidation process

STP

Sewage treatment plant

WWTP

Wastewater treatment plant

XPS

X-ray photoelectron spectroscopy

Contamination of water bodies by ‘emerging pollutants’ (EPs) has become a huge problem in the recent years (Gogoi et al. 2018; Rathi et al. 2021; Scaria et al. 2021a, 2021b). One such emerging problem is the contamination by pharmaceuticals and personal care products (PPCPs) which has created a major problem by degrading the water quality and endangering the aquatic ecosystem (Dey et al. 2019; Patel et al. 2019; Ngqwala & Muchesa 2020; Mohan et al. 2021). PPCPs also constitute active pharmaceuticals ingredients (APIs) of prescribed, non-prescribed and illegal drugs for human, plant and veterinary use (Dey et al. 2019).

Major pharmaceutical contamination in water bodies arise mainly due to unused medicines from hospital waste and human excretion via urine and feces. This is because the drug and their metabolites are partially consumed in the human body (Taylor & Senac 2014; Patel et al. 2020; Scaria et al. 2021a, 2021b). PPCPs in the form of APIs have also been shown to generate from a small molecule into at least 20,000 varieties of drug products (Evgenidou et al. 2015).

Conventional wastewater treatment systems that involve physical, chemical and biological removal processes are not efficient to treat PPCPs completely due to their persistent nature. Advanced oxidation processes (AOPs) are steadily emerging as a potential chemical technology that primarily relies on partial or complete oxidation of several recalcitrant compounds by the generation of free radical species (^•OH and ⁠), which result in the breakdown of pollutants (both organic and inorganic) into more fundamental and non-toxic molecules (Kanakaraju et al. 2018; Babu et al. 2019; Nidheesh et al. 2022a). Various AOP methods such as ozonation, Fenton, Photo-Fenton and UV/H₂O₂ processes have been previously applied as suitable alternative chemical processes for tertiary or post-tertiary water and wastewater treatment in degrading PPCPs (Anjali & Shanthakumar 2019; Krishnan et al. 2021; Scaria & Nidheesh 2022c). Furthermore, AOPs in combination with porous adsorbents (activated carbon (AC), graphene) have been a popular technology to be used as catalysts, which increase the degrading efficiency of several PPCPs in aquatic systems by enhancing the generation of free radical species in the system (Nidheesh 2017; Rubeena et al. 2018; Nidheesh et al. 2021; Scaria et al. 2022).

Among these materials, biochar (BC), a carbon-centered material obtained from plants such as bamboo, wheat straw (He et al. 2021) and animal biomasses like swine bone (Zhou et al. 2021) and shrimp shell (Yu et al. 2020), has gained importance for making supported catalysts in AOPs. There is a broad interest in the applications of BC in AOPs due to its combined ability as an adsorbent as well as a catalyst (Nidheesh et al. 2021; Nakarmi et al. 2022). BC has been applied to soil in agriculture to increase its nutrient content, thereby increasing the fertility of soil for enhancing crop yields, as an adsorbent material for removal of organic pollutants and also studied as a potential CO₂ mitigation material (Mohan et al. 2014; Ding et al. 2016; Lee et al. 2017). This is attributed to its physicochemical properties such as a high surface area, porosity, cation exchange capacity and abundant surface functional groups (SFGs) (Ni et al. 2020; Tomczyk et al. 2020). These groups can be functionalized suitably by altering the pyrolysis temperature and the feedstock source.

There is a growing trend in the applications of BC as a catalyst in AOPs for effective degradation of toxic-pollutants, including endocrine-disrupting chemicals (EDCs) and persistent organic pollutants (POPs). Studies have shown that BC contains persistent free radicals (PFRs), alternatively termed as ‘Environmentally Persistent Free Radicals’ (EPFRs), which help to enhance free radical generation in AOPs resulting in the degradation of several PPCPs (Nidheesh et al. 2021; Kumar et al. 2022; Scaria & Nidheesh 2022a).

Recently, BC as a catalyst in combination with AOPs that particularly generate (SR-AOPs) is becoming a popular method for the degradation of PPCPs from wastewater (Guerra-Rodríguez et al. 2018; Delgado et al. 2020; Ushani et al. 2020). Preference for over the conventional ^•OH is that has a higher oxidation potential (2.5–3.1 V), longer half-life (30–40 μs), can support a broader range of pH (2.0–8.0) and has better non-selectivity for degrading organic contaminants compared with ^•OH (Nidheesh & Rajan 2016; Ushani et al. 2020; Kohantorabi et al. 2021; Gujar et al. 2022; Scaria & Nidheesh 2022b). This review focuses on summarizing the role of BC as a supported catalyst in SR-AOPs in removal of PPCPs. The catalytic ability of the BC will be stressed and its possible utilization and major challenges in real wastewater treatment for treatment of PPCPs will be discussed.

PPCPs, due to their increasing applications as medicines, have received much attention for their contamination in aquatic ecosystems because of their persistent nature even at very low concentrations (ng/L–μg/L). PPCPs mainly constitute antibiotics, anti-inflammatories, analgesics, anticonvulsants, antidepressants, lipid regulators, ß-blockers, artificial sweeteners and personal care products such as sunscreens, body lotions, sprays and musks (Wang & Wang 2016). Exposure of PPCPs to humans can occur mainly through ingesting of packaged/canned food, breathing contaminated air and interacting with contaminated soil whereas the effluent streams from the WWTPs are the main exposure to the aquatic ecosystem. There are no concrete experimental studies for the direct toxic effects of exposure of PPCPs to humans (Wilkinson et al. 2016). However, in a research work by Hind M. Ewadh and group, exposure of PPCPs (ibuprofen (IBU), ketoprofen) on the cells of human embryonic kidney (HEK 293) were studied. About 90% reduction of cell proliferations in the kidney were observed due to the increased exposures to high concentrations of IBU (750 μg/L) and ketoprofen (75 μg/L), indicating the extensive sensitivity of cells in human organs toward increased chemical dosages (Ewadh et al. 2018).

The presence of trace amounts (ng/L) of PPCPs greatly induces toxic effects on ecological systems by harming their psychological and hormonal responses. Joon-Woo Kim and group studied the toxicity effects of pharmaceutical chemicals such as IBU and carbamazepine (CBZ) on freshwater crustaceans (Thamnocephalus platyurus) and fish (Oryzias latipes) and observed that the 24- and 96-h median lethal concentration (LC₅₀) levels of IBU (19.59 mg/L) and CBZ (81.92 mg/L) affected the central nervous systems, thereby decreasing the overall neuronal activity of the species (Kim et al. 2009).

Similarly, antibiotics such as ofloxacin, lincomycin and ciprofloxacin (CIP) are known to be toxic to freshwater algae. Low concentrations (ng/L) of oestrogen 17-alpha-ethinyloestradiol have been shown to cause abnormalities in fish livers and affect sexual characteristics in male fish (Ngqwala & Muchesa 2020). It is reported that the major exposure of PPCPs to the aquatic flora and fauna are through the effluents discharged into the water bodies (runoff, rivers and floods) (Balakrishna et al. 2020).

Current PPCPs removal processes in WWTPs and STPs are adsorption and biodegradation. Biodegradation is not effective to mineralize or breakdown PPCPs completely due to the persistency nature of PPCPs, and its toxicity to microorganisms (Wang & Wang 2016). Moreover, the decreased removal efficiency of chemicals such as diclofenac and carbamazepine in the activated sludge process of WWTPs and STPs was also evident with only 30% of carbamazepine and 10% of diclofenac removal observed in the WWTPs processes (Zhang et al. 2008). This signifies the inefficiency of the conventional wastewater treatment systems to degrade PPCPs effectively.

Furthermore, only a particular category of PPCPs called fluoroquinolones were effectively removed in STPs through biodegradation and adsorption (Lin et al. 2010; Lin & Gan 2011; Balakrishna et al. 2020). Other categories of antibiotics such as macrolides, sulfonamides, penicillin and imidazole showed no effect in treatment. A total of 71 pharmaceuticals in the districts of Shanghai were also monitored in a study by Liu and group. About 10 PPCPs were detected in very high concentrations, which was attributed to the wastewater treatment plants in the vicinity (Liu et al. 2019). Table 1 shows the removal rates of different PPCPs from STPs and WWTPs treatment processes.

The occurrences of PPCPs due to their persistent nature have also been observed in sediments, soil strata, rivers and lakes. PPCPs such as triclosan can adsorb onto sediments and may release back into the water environment (Liu & Wong 2013). Anti-inflammatory drugs such as diclofenac and IBU result in incomplete sorption onto surface soils. Moreover, the incomplete sorption permits its leaching into the groundwater during irrigation or surface spreading of treated wastewater, which increase the risk of groundwater contamination (Lin & Gan 2011).

Studies have mentioned that reclaimed wastewater used for agricultural purposes and biosolids along with wastewater treatment byproducts in agricultural areas are two of the major sources of PPCP contamination in soils. Most commonly detected compounds in sludge and biosolids are carbamazepine, diclofenac and IBU. All these compounds are known to have a lower sorption potential which restricts its adsorption onto organic matter. Concentration levels of antibiotics such as clarithromycin (235 ng/L), erythromycin (320.5 ng/L), metronidazole (1,195.5 ng/L) and SMX (326 ng/L) were predominantly found in the lakes of Jarma, Manzanares, Guadarrama, Henares and Tagus in Spain (Valcárcel et al. 2011). Similarly, rivers such as Pearl, Yellow, Hai and Liao in China have detected antibiotic concentrations of norfloxacin (NOR) (5,770 ng/L), ofloxacin (OFX) (1,290 ng/L) and CIP (653 ng/L) (Peng et al. 2008, 2009, 2011).

Higher levels of PPCPs have also been found in sludge. A study by Huber and group observed high concentrations of cetrimonium salts (680 μg/L) in the sludge of the Icelandic wastewater treatment plant and concluded that sludge could act as sinks for PPCP accumulation (Huber et al. 2016). Moreover, higher concentration levels of norfloxacin (6,490 ng/g) and CIP (6,050 ng/g) were detected in sediment samples collected from an urban drain (Ashfaq et al. 2019). Table 2 shows a summary table depicting the concentration levels of some PPCPs in different environments.

PPCPs have also been found in plant and animal tissues, indicating their bioaccumulative properties and their occurrence are said to be influenced greatly by the efficiency of the wastewater treatment plants. Plants absorb the minerals and nutrients from the soil through the uptake process. N4-acetyl-sulfamethoxazole (96 ng/g), sulfamethoxazole (SMX) (24 ng/g) and triclosan (18 ng/g) had been predominantly detected in the crops grown from an agricultural system in China irrigated by reclaimed wastewater (Reyes et al. 2021). A global scenario on the concentration of different PPCPs in STPs and WWTPs at different locations has also been listed in Supplementary Table S1.

SR-AOPs have also been applied for groundwater and soil remediation, specifically because sulfate radicals have higher stability and can travel through subsurface gradients in the soil efficiently compared with hydroxyl radicals (Duan et al. 2020). SR-AOPs in the presence of nitrite to remove phenolic substrates from groundwater and wastewater have been investigated as a potential in-situ chemical oxidation treatment. Thermally operated PS oxidation of phenol in the presence of can lead to the formation of nitrophenol byproducts such as 2-nitrophenol (2-NP), 4-nitrophenol (4-NP), 2,4-dinitrophenol (2,4-DNP) and 2,6-dinitrophenol (2,6-DNP). The degradation of phenols was attributed to the generation of NO₂^• from the scavenging of and ⁠. It was also concluded from the studies that even though generation of NO₂^• results in the degradation of phenols, the formation of nitrophenol byproducts during implementation in SR-AOPs needs control measures as such nitrated aromatic compounds can be carcinogenic, mutagenic and genotoxic to the aquatic organisms (Ji et al. 2017).

SR-AOPs in removing pharmaceuticals such as bisphenol-A by thermally activated PS show its credibility to degrade not only refractory organics but also PPCPs from wastewater. The performance of SR-AOPs greatly increases with the addition of a catalyst by enhancing the generation of sulfate radicals. Metal-based catalysts such as metal ions (cobalt, silver and manganese) are known to donate electrons to generate sulfate radicals in PS/PMS systems.

Carbonaceous materials such as graphene, AC and BC are said to contribute to generation via the formation of π-electrons upon interacting with SFGs (–COOH, –OH, C = O) (Nidheesh et al. 2021; Kumar et al. 2022). The catalytic ability of BC-supported catalyst in SR-AOPs and its functionalization to improve surface properties for the degradation of PPCPs have been discussed in subsequent sections.

BC has been widely accepted as an adsorbent to majorly transfer pollutants from one medium to another (Ahmad et al. 2014; Ghaffar et al. 2018). This exceptional behavior of BC is attributed to its morphological structure which comprises aromatic carbon and condensed aromatic structures (Mohan et al. 2014; Tomczyk et al. 2020). BC as a potential heterogeneous catalyst has been demonstrated in biodiesel production, removing tar in bio-oil and syngas and serving as far as electrode material in fuel cells (Lee et al. 2017). Moreover, its participation as a catalyst has gathered wide attention recently because of the presence of abundant oxygen-containing functional groups (OFGs) which comprise of carbonyl (C = O), carboxyl (–COOH) and hydroxyl (–OH) moieties that contribute to the degradation of target pollutants. The presence of OFGs again strongly depends on the pyrolysis temperature, residence time and the feedstock of the material used (Faheem et al. 2020). These OFGs, upon photo-based irradiation, generate reactive oxygen species (ROS) (^•OH, ⁠, ¹O₂ and ⁠) through an electron transfer mechanism (Luo et al. 2021). This results in the interaction of OFGs on the surface of the BC with the ROS.

From the pyrolysis of biomass, large groups of phenol or quinone moieties are produced from the lignin in biomass for transferring electrons to transition metals, which leads to the formation of PFRs in BC (Wang et al. 2019c). The existence of PFRs in BC is the main factor for the degradation of complex species and has attracted interest in exploring its catalytic abilities. PFRs in BC are generated from the incomplete carbonization of organic substances resulting in the formation of ‘dangling bonds’. These bonds form complex structures with metal oxides to form stabilized radicals such as semiquinones, cyclopentadienyls and phenoxyls (Luo et al. 2021).

PFRs are categorized into two types, namely: oxygen-centered and carbon-centered radicals. Oxygen-centered radicals mainly consist of semiquinolone free radicals and are present up to a pyrolysis temperature of 300 °C. When the temperature further reaches 400 °C, carbon-centered radicals are formed with adjacent oxygen. Carbon-centered radicals usually have better electron-donating capacities and are responsible for radical (^•OH and ⁠) generation in SR-AOPs. Optimum generation of PFRs in BC from coconut shell, eucalyptus leaves and walnut shell were observed at a pyrolysis temperature of 500 °C and result in optimum generation of sulfate radical in AOPs (Zhang et al. 2021). The formation of PFRs in BC at different stages of pyrolysis of biomass has been shown in Table 3.

Table 3

Different thermal stages of biochar synthesis in relation to activation (Odinga et al. 2020)

Sr.no .	Pyrolysis temperatures of biochar (°C) .	PFR production in biochars .	Reasons .
1	0–300	Little to no radical production	No alteration in overall structure of biomass with minute mass loss due to loss of water elimination, bond breakage (not preferred for activation)
2	300–500	Oxygenated and carbon-centered radical generation	Conversion of oxygen-centered radicals into carbon-centered radicals (most preferable for activation)
3	500–700	Gradual reduction of PFRs	Production of graphene-like sheet structures

Sr.no .	Pyrolysis temperatures of biochar (°C) .	PFR production in biochars .	Reasons .
1	0–300	Little to no radical production	No alteration in overall structure of biomass with minute mass loss due to loss of water elimination, bond breakage (not preferred for activation)
2	300–500	Oxygenated and carbon-centered radical generation	Conversion of oxygen-centered radicals into carbon-centered radicals (most preferable for activation)
3	500–700	Gradual reduction of PFRs	Production of graphene-like sheet structures

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Biomass consists of cellulose, hemicellulose and lignin. The decomposition of cellulose and hemicellulose in the biomass occurs at a pyrolysis temperature above 300 °C, leaving behind lignin residues. Hence, the formation of PFRs in the BC is influenced with the presence of lignin content (Kibet et al. 2012; Luo et al. 2021; Zhang et al. 2022).

In summary, the PFRs help in the generation of by the activation of PS/PDS and PMS in SR-AOPs. The performance of the catalytic ability of the BC is strongly influenced by its pyrolysis temperature and the feedstock material used. The OFGs on the BC surface activates the PS and PMS by generating ROS due to electron transfer between BC surface and PS in the aqueous degradation of the PPCPs. Hence, it is required to maintain adequate pyrolysis temperature for the optimum generation of PFRs in the BC.

ROS generated from the activation of PS/PDS or PMS result in two pollutant degradation pathways: radical (^•OH and ⁠) and non-radical (¹O₂). Degradation of pollutants via non-radical pathway involves singlet oxygen (¹O₂), electron transfer and bridging of carbon catalyst for facilitating electron transfer from contaminants to PS/PDS or PMS. Studies have also referred to BC as an ‘electron shuttle’ which depicts the transfer of electrons as a result, directly affecting the degradation pathway (Klüpfel et al. 2014; Faheem et al. 2020). Studies on the non-radical pathway carry several merits such as complete utilization of the oxidizing capability of PMS and segregation of organic/inorganic contaminants in the selected matrix (Kohantorabi et al. 2021). Zhou et al. investigated the catalytic performance of bone-derived BC pyrolyzed at 1,300 °C for degrading acetaminophen (ACT). The presence of the ketone (C = O) group facilitated the electron transfer mechanism by generating singlet oxygen (¹O₂) by the non-radical pathway which then acted as active sites for effective degradation of ACT. Second, carboxyl (–COOH) and hydroxyl (–OH) groups proved beneficial for generating ^•OH and via the radical pathway (Zhou et al. 2020, 2021).

In additional studies, a comparison between two heterogeneous Fe-based catalysts (bentonite-combined Fe–Ni and BC-combined Fe composite) for the degradation of sulphapyridine (SPY) and oxytetracycline (OTC) demonstrated that the BC/Fe catalyst initiated singlet oxygen (¹O₂) generation, which resulted in increased degradation rate of SPY and OTC (Li et al. 2020c). The contribution of radical and non-radical pathways was investigated in a study by Zou et al. where metal-free BC catalyst made from corn cob was investigated for removing five PPCPs from pharmaceutical wastewater. The higher contribution for the PPCPs degradation was shown by the radical pathway which was governed by the carbon framework (sp² hybridized carbon) of the BC to produce free radicals. In contrast, activation into ¹O₂ by the ketone group (C = O) was responsible for the non-radical pathway, both of which were achieved at a pyrolysis temperature of 800 °C (Zou et al. 2020).

Surprisingly, contradictory results regarding the formation of the oxidized groups (C = O, –COOH, –OH) were also observed in accordance with the activation of BC for sodium persulfate (SPS) treatment through the non-radical pathway (Ntzoufra et al. 2021). Fourier transform infrared spectroscopy (FTIR) observations showed the formation of sulfonic groups (–SO₃H) on the surface of the BC. This was due to the oxidation property of ⁠. It was also observed that the peak intensities in the FTIR spectrum showed that the presence of –C = O groups gradually increased upon PS oxidation, pointing out the oxidation of the –C–C and –C = O groups.

Additionally, X-ray photoelectron spectroscopy (XPS) analysis of the BCs also showed that higher binding energies were ascribed to the oxidized C groups (–C = O or O–C = O). The amount of non-oxidized C species in the BC was higher before PS oxidation, whereas after treatment, formation of oxidized C species (C = O) was more predominant. This was also further validated by Raman spectroscopy where the peak (at 1,233 cm⁻¹) was more intense with the absence of oxygen-containing groups. In the results reported, this peak which represents sp³ hybridized (–CH₃ group) structures diminished upon PS oxidation indicating the enrichment by oxygen. The results concluded that although BC was a stable catalyst for the oxidation of organic contaminants, the effectiveness of PS treatment for longer periods could result in the oxidation of functional groups on the surface of the BC which is a main deactivating factor for its catalytic ability (Ntzoufra et al. 2021).

Functional BCs are becoming an upcoming sustainable material for the remediation of pollutants, particularly for removing antibiotics from wastewater systems through the adsorption mechanism (Muter et al. 2019; Krasucka et al. 2021). Engineered BC was initially applied for soil improvement; for example, BC of high porous nature is preferred to enhance the crop yield. Biochar, due to its high organic carbon, nitrogen and ash content can increase the soil organic carbon (SOC), which is a health indicator of soil fertility (Kazemi Shariat Panahi et al. 2020). Pristine BC has a lower density and smaller particle size, making it difficult to separate from water and limiting its applications (Tan et al. 2016; Wang et al. 2017).

Engineered BCs have been investigated for removing pharmaceuticals such as diclofenac, naxoprofen and triclosan from water medium. Higher adsorption rates were achieved for the selected PPCPs on the engineered BC. Low pyrolysis temperature favored increased adsorption of PPCPs due to the pore filling mechanism and hydrophobicity of the chemicals (Czech et al. 2021).

Another study on woodchip-derived BC pyrolyzed at a temperature of 725 °C for adsorbing PPCPs such as xylazine, metoprolol, azithromycin, ketoprofen, diclofenac, IBU, sulfamethoxazole (SMX) and simvastatin indicated that the adsorption followed by incubation could provide better results for some selected pharmaceuticals. However, this was majorly dependent on the sorption capability of the compounds (Muter et al. 2019). Modification of BC through addition of a methanol solvent can alter the SFGs or oxygen-containing groups of the BC to adsorb tetracycline (TCE) (Jing et al. 2014). Engineered BCs are typically used as a tool to enhance the adsorption capabilities which are governed by the selected contaminant and the production method. Results suggest that BC-based sorbents, pristine BC produced at different conditions, modified BCs and BC composites are effective in the removal of certain antibiotics (Krasucka et al. 2021). The effectiveness depends on the physicochemical properties of both antibiotic and adsorbent. The binding of antibiotic onto the BC surface is dependent on the surface functional group, pH and the carbon content of the BC as well as the sorption parameters of the target pollutants (Log K_ow). The phenomena of pore filling mechanisms have also been stressed (Krasucka et al. 2021).

Engineered BC have been well established as adsorbents that increase the removal rate of many PPCPs. The modification of BC can alter its morphological structure, thereby producing functional groups on the BC surface. Different modification methods increase the micropores of the BC which in turn increases the active sites of the BC for better interaction toward target pollutants. Hence, proper modification of the BC surface can result in better generation of PFRs in the BC which can enhance the radical generation in SR-AOPs.

SR-AOPs by PS activation have wide applications in the degradation of antibiotics from synthetic wastewater such as tetracycline (TCE) (Zhong et al. 2020), CIP (Gao et al. 2020), SMX (Avramiotis et al. 2021) and cephalexin (CEX) (Song et al. 2021). All the studies included the identification of reaction pathways and mechanisms through quenching experiments and electron paramagnetic resonance (EPR) analysis. Zhong and group studied the removal of TCE using BC co-doped with metals. The degradation of TCE by the nitrogen and copper co-doped BC (N–Cu/BC) succeeded even after reusing with efficiencies reported up to 80 and 65% on the 2nd and 3rd times indicating the increased catalytic ability of the metal-doped BC. EPR studies also confirmed the generation of and ^•OH. The ^•OH played a crucial role in the degradation of pollutants. Moreover, the non-radical pathway was attributed to the electron transfer between TCE and PS at the surface of the BC catalyst (Zhong et al. 2020).

SR-AOPs to generate sulfate radicals have also been increasingly applied to treat synthetic groundwater matrices. Selectivity of PS/PDS is attributed to its low price, high stability and easier transport in the subsurface environments compared with PMS (Duan et al. 2020). BC derived from maize stock and cob pyrolyzed at different temperatures (300 and 600 °C) were examined for the degradation of TCE from groundwater, superoxide and singlet oxygen along with ketone (C = O) groups were the main ROS and OFGs influencing the degradation rate of TCE. However, it was also noted that the presence of other anions in the groundwater matrix (chloride (Cl⁻), nitrate (⁠⁠)) acted as scavengers in restricting the generation of sulfate radical thereby reducing the degradation rate of TCE. Inhibiting of ROS generation ability was also observed due to the reaction between Fe species with phosphate anions (Li et al. 2020b).

BC/Fe-activated PS treatment was applied by (Wang et al. 2019b) for the degradation of SMX. A BC-induced Fe(III) reduction system efficiently degraded SMX due to the presence of PFRs in BC which facilitated electron transfer between the BC surface and Fe(III). Fe(II) was generated for activation of PS. Tert-butanol was responsible to inhibit the degradation of SMX by hindering the electron transfer between BC and Fe(III). Iron and its oxide are widely used for their nontoxicity, low cost and eco-friendly nature (Wang et al. 2019a, 2019b, 2019c). Pristine BC derived from coconut shell has also been investigated for the removal of SMX from an aqueous solution. About 85% removal was achieved which was attributed to the generation of ROS (⁠⁠, ^•OH) and ¹O₂ by PMS activation through the non-radical pathway (Hung et al. 2022). Bo Wang and group did a combined study of norfloxacin (NOR) removal in deionized water using corn stalk biochar (CBC) at a pyrolyzed at 500 °C. The combination of CBC, PS and CBC/PS systems were applied for the degradation of NOR. The rate of NOR increased with the increase of CBC dosage. This was effective in catalyzing PS consequently leading to increased generation of ultimately leading to the acceleration of the reaction (Wang et al. 2019a, 2019b, 2019c). Different pharmaceuticals and their removal using BC as a catalyst in SR-AOPs are listed in Table 4.

SR-AOPs by activation of PS/PDS have been focused due to the stability of at ambient conditions followed by strong water solubility of PS. PMS requires higher energy to produce radicals due to a larger dissociation energy (377 kJ mol⁻¹) to break the peroxide O–O bond length (1.437 Å) compared with PDS (1.497 Å and 97 kJ mol⁻¹) (Flanagan et al. 1984; Nidheesh et al. 2022b). Degradation of SMX and its human metabolite N4-acetyl-sulfamethoxazole (NSMX) in urine by PDS combined with BC derived from cotton straw biomass under oxygen-free conditions at 350 °C was studied (Zhang et al. 2020). The results concluded that the BC-aided technology sufficiently degraded SMX and NSMX in the urine matrix with minor interventions due to formations of HCO₃̄ and NH₄⁺. Synergistic effects of ^•OH generation and singlet oxygen species were the dominant mechanisms in the degradation (Zhang et al. 2020).

In the study for CIP degradation, sulfide and nanoscale zero-valent iron-doped BC (S-nZVI/BC) observed a similar influence of and ^•OH as the primary ROS for the process (Gao et al. 2020). However, in the case of SMX removal by rice husk-derived BC, a comparison between spiked samples with bicarbonates and real wastewater indicated that ionic species (Cl⁻, Br⁻) in real wastewater inhibit the performance of the BC catalyst (41% against 51% in 90 min) (Avramiotis et al. 2021). Li and group conducted a similar study by applying a composite of BC-supported by copper oxide (CuO) for removal of pharmaceuticals such as ciprofloxacin and atrazine from highly saline wastewater by PMS activation. It was observed that ciprofloxacin was completely removed (100%) followed by atrazine (78.27%). A comparison between synthetic salt water system and actual saline wastewater showed no significant changes in the removal rates of the antibiotics. The non-radical pathway was the dominant pollutant degradation mechanism. The influence of OFGs was also responsible for the generation of ¹O₂ (Li et al. 2020a).

Phenolic byproducts are also a result of industrial effluent discharge of pharmaceuticals and other detrimental sources. BC modified by lanthanum chloride effectively degraded phenol by PS activation with efficiencies reaching about 97%. Phenol concentration, pH, catalyst dosage as well as PS addition governed the activation of PS. It was observed that the optimum removal was achieved at a pH of 4.0. This was attributed to the surface charge of the BC catalyst as the +ve charge of BC surface attracts the −ve charge phenolate anion leading to subsequent removal of phenol (Razmi et al. 2019). Bai et al. (2022) studied the effect of chitosan-based BC for PS activation to degrade phenol from saline wastewater. The non-radical pathway using ¹O₂ was the predominant pollutant degradation mechanism followed by radicals (⁠ and ^•OH). The presence of NaCl added to produce a saline media had an inhibitory effect on the phenol degradation and increased with additional dosage (Bai et al. 2022). Bo Wang and group also studied the effect of aquatic systems on the degradation of norfloxacin (NOR) by PS activation using BC obtained from corn stalk. Groundwater and surface water samples from Liaoning province and Hunhe river showed that PS activation by BC as catalyst was negligibly affected and catered successful degradation of NOR. The pH conditions of the aquatic sources indicated that a slightly acidic medium favored the degradation of NOR due to the formation of and ^•OH but also depended on the PS dosage (Wang et al. 2019a, 2019b, 2019c).

In a similar study, the applicability of egg shell-derived biochar at a pyrolysis temperature of 600 °C (ES-biochar) by PS activation for removal of 2,4-dichlorophenol in real water sample of Taozi Lake in China was investigated (Liu et al. 2020). Up to 88% efficiency was achieved for about 2 h, although the removal rate was slower compared with the results obtained by aqueous solutions. This was attributed to the presence of other organic compounds in the lake water.

Overall, the superiority of the BC-supported catalyst has been well established as far as controlled environments are concerned. The effect of BC as a catalyst material shows positive results in completely degrading PPCPs from the aqueous medium. The above studies show that the amount of catalyst as well as PS and PMS dosage is largely dependent on the target pollutant to be removed. However, the suitability of BC-supported catalyst in SR-AOPs in real applications are fairly limited. One reason is that SR-AOPs are associated with high generation of sulfate ion which can breach the regulation limits (500 mg/L). Moreover, adequate analytical techniques such as EPR, free radical quenching experiments and density functional theory (DFT) calculations are needed to identify the radicals generated and the degradation pathways of pollutant degradation (Duan et al. 2020). Acidic pH environments are shown to be favorable in most studies as the acidic nature helps in the quicker generation of in the system.

Although metal-based catalysts are well known for activation of PDS and PMS in SR-AOPs, there are certain limitations which can pose a challenge and can be avoided by using BC-based catalysts:

• (1)

  Application of transition metals or their oxides are generally reduced by excessive leaching of metal ions which can increase the overall pH of the aqueous medium (Kohantorabi et al. 2021). BC-based catalyst are found much effective for the degradation pollutants with lesser metal leaching (Delgado et al. 2020; Zhao et al. 2021).

• (2)

  The metal ions can form secondary toxic intermediates upon reacting with existing ions in the contaminated water and pose a challenge for the aquatic health. BC-based catalysts are said to reduce the agglomeration of transition metals by ensuring their uniform distribution and can accelerate the transfer of electrons due to the presence of free radical species in the BC matrix (Yu et al. 2019; Wang et al. 2021).

• (3)

  The synergistic advantages of modified biochar both as an adsorbent and a catalyst material can be accessed in-situ for the removal of antibiotics from water medium. A study reported the dual applications of corn stalk biochar pyrolyzed at 700 °C to remove TCE from water medium. Adsorption of TCE was credited to the increased surface area of the BC whereas the reduction of Cu(I) on the BC resulting in generation of and ^•OH by using the PS activation method was responsible for the increased catalytic ability of the BC for degrading TCE (Chen et al. 2020).

Although there are many studies conducted on BC as a catalyst in SR-AOPs, the applications are limited to laboratory-scale analysis. The implementation of BC as a catalyst in SR-AOPs for real systems/scale has potential challenges and their possible mitigation can be summarized in the following points:

• (1)

  Adsorptive abilities of BC are more pronounced for its application in removal of organic pollutants due to its large surface area, SFGs and porosity. Applying BC catalyst for SR-AOPs can initiate dominance of adsorption over its catalytic ability for activation of radicals. Moreover, BC produced at high pyrolysis temperature reduces the presence of PFRs required for activation of PS but favors better surface properties of the BC to adsorb pollutants. On the contrary, BC produced from low pyrolysis temperatures facilitate high amount of PFRs but caters to reduced surface properties for adsorption. This can be somewhat minimized by producing BC at adequate temperatures (between 300 and 500 °C) to enhance the combined abilities of adsorption as well as a catalyst.

• (2)

  Another challenge can be the subsequent poisoning or deactivation of the surface of the BC catalysts due to the adsorption of intermediates or byproducts during the degradation process. This can potentially reduce the efficiency of BC due to the accumulation of toxic intermediates into the active sites of the BC surface and can create a challenge for producing tailored BC (adequate surface properties as well as sufficient PFR generation) for producing ⁠.

• (3)

  Implementation of SR-AOPs in real wastewater containing ions such as chloride (Cl⁻) and bromide (Br⁻) are said to form unwanted toxic intermediates like chlorate (⁠⁠) and bromate (⁠⁠) by the generation of ⁠. This can be prevented by adjusting the pH or chemical addition to suppress the formation of XO₃⁻. Lower pH (below 5) promoted Cl^• to form whereas pH > 5 resulted in the formation of ^•OH due to the reaction between Cl^• and water (Faheem et al. 2020).

Here, a summarization on BC-supported catalysts and their functionalization products for their increased degradation efficiencies of PPCPs in SR-AOPs has been made. The presence of PFRs in the BC helps in the generation of radical species in SR-AOPs which result in better degradation of pollutants when BC is used as a catalyst. Pyrolysis temperature and feedstock material play a vital role in the efficiency of the BC to be used as a catalyst.

BC-supported catalyst can degrade PPCPs in SR-AOPs through radical (^•OH, ⁠) and non-radical (¹O₂ and electron transfer) pathways which is influenced by the surface properties such as the SFGs on the surface of BC. For increasing the catalytic ability of the BC by the non-radical pathway, formation of oxidized carbon groups can pose a problem for activating PS and PMS in SR-AOP treatments as they can alter the active sites of the BC surface and deactivate the BC in the long run which can pose a challenge for the in-situ applications of the BC-supported catalyst. Suggestions to reduce the negative influence of oxidized groups on the BC surface by the non-radical pathway of degradation in SR-AOPs may require further studies.

Overall, BC-supported catalyst in SR-AOPs is a promising technology for removing PPCPs from water bodies. Feasibility studies on field implementations require a cost-benefit analysis and the long-term efficiency of BC-supported catalyst needs further evaluation.

The authors are thankful to the Director, CSIR-NEERI, Nagpur, India for encouraging and kind permission for publishing this article.

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

The authors declare there is no conflict.