Photocatalytic Composites Based on Biochar for Antibiotic and Dye Removal in Water Treatment

Abstract

Many semiconductor photocatalysts are characterized by high photostability and non-toxicity but suffer from the limited excitation in the UV part of the spectrum and the fast recombination of the photogenerated electron–hole pairs. To improve the above properties, biochar-supported composite photocatalysts have recently attracted much attention. Compared with the pure photocatalyst, the biochar-enriched catalyst has superior specific surface area and high porosity, catalytic efficiency, stability, and recoverability. Biochar can be obtained from various carbon-rich plant or animal wastes by different thermochemical processes such as pyrolysis, hydrothermal carbonization, torrefaction, and gasification. The main features of biochar are its low price, non-toxicity, and the large number of surface functional groups. This paper systematically presents the latest research results on the method of preparation of various composites in terms of the choice of photoactive species and the source of biomass, their physico-chemical properties, the mechanism of the photocatalytic activity, and degradation efficiency in the treatment of organic contaminants (dyes and antibiotics) in an aquatic environment. Particular emphasis is placed on understanding the role of biochar in improving the photocatalytic activity of photoactive species.

1. Introduction

Although 70% of planet Earth is covered in water, only 2.5 percent is fresh water that we drink and use for irrigation, the rest being salt water and oceans. Only 1% of fresh water is readily available, and most is trapped in glaciers and snowfields. In fact, only 0.007% of the water on the planet is available to feed 6.8 billion people. According to the United Nations, by 2025, an estimated 1.8 billion people will live in water-scarce areas, and two-thirds of the world’s population will live in water-scarce regions as a result of use, growth, and climate change [1]. Inadequate sanitation is also a problem for 2.4 billion people—they are exposed to diseases such as cholera and typhoid and other water-borne diseases. Two million people, mostly children, die every year from diarrhea alone [2]. Increasing consumption, the accelerated pace of urbanization and industrialization, and the depletion of clean water sources are just some of the reasons for the shortage of drinking water [3].

Population growth, improved living standards, and economic growth were accompanied by the creation of 380 billion m³ per year of municipal wastewater. It is estimated that wastewater production will increase by 51% by 2050 [4]. Although the percentage varies by location, it is believed that over 80% of wastewater produced worldwide is released into the environment without receiving proper treatment [5]. High-income nations treat, on average, 70% of the wastewater they produce, according to UN Water [6]. In nations with higher middle incomes, this ratio falls to 38%, while in nations with lower middle incomes, it reduces to 28%. Merely 8% of wastewater generated in low-income nations receives any form of treatment.

Global environmental problems include antibiotic pollution and the growth of microbes resistant to antibiotics. The market for antibiotic production was valued at USD 40.70 billion in 2021 and is projected to grow at a compound annual growth rate (CAGR) of 5.25% from 2022 to 2029, according to Data Bridge Market Research’s analysis [7]. Besides pharmaceutical wastewaters, the other concerning pollutants are dyes from the textile industry. The global dyes and pigments market size is estimated to be valued at USD 39,160 million in 2022 and is projected to have an adjusted size of USD 49,830 million by 2028 [8] with an annual growth rate (CAGR) of 5.3% from 2023 to 2030 [9]. China is the largest dyes and pigments market with about 41% market share, followed by Europe with about 19%, respectively [8].

According to the report “Progress towards the Sustainable Development Goals” [10] from 2024, as of 2022, 2.2 billion people did not have access to safely managed drinking water, and 3.5 billion did not have access to safely managed sanitation. The fact that 80 percent of wastewater in the world flows back into the ecosystem without being treated or reused is worrying. According to this report, an increase in the use of potable water for drinking by 4% between 2015 and 2022 has been recorded.

Given the huge amount of antibiotics and dyes that are produced globally and that a good part of them ends up in wastewater or improperly disposed of, this study deals with the possibility of their removal using photocatalytic composites based on biochar. Both financial and environmental gains can be obtained from the sorption of contaminants using “green”, inexpensive, or sustainable biochar. Furthermore, this is consistent with worldwide tendencies toward the creation of sustainable development and a circular economy as well as preventing the emission of greenhouse gases during the conversion of organic biomass into carbon and long-term carbon storage [11].

The roles of biochar as a support for photoactive species are multiple, and some of them include increasing the surface area and number of active sites, shuttling electrons, serving as an electron reservoir, improving charge separation, and reducing bandgap energy [12]. Due to its high contaminant adsorption properties, biochar can be a very efficient and sustainable choice for industrial water filtration, solid waste treatment, and wastewater treatment. Herein, two main processes are very important: adsorption and photocatalysis. Adsorption is regarded as one of the most straightforward, dependable, affordable, and effective strategies for treating wastewater in the conventional sense. Likewise, photocatalysis is regarded as an innovative oxidation technique for wastewater treatment that is green, efficient, safe for the environment, and promising. The challenge we face now as we move into the future is how to effectively conserve, manage, and distribute the water we have. The process of wastewater treatment involves different steps that work in tandem to remove pollutants using a combination of chemical, biological, and physical processes. It is not a one-step operation. Nowadays, heterogeneous photocatalysis is widely studied using various methods of heterogenization of semiconductor materials for water treatment. One of the most studied semiconductors is TiO₂. In addition, ZnO, CdS, and many others have also been investigated. Various modification methods are used to improve their photocatalytic activity, such as doping with metals, metal oxides, non-metals, and others. By combining semiconductors with carbon materials, it is possible to influence the electronic properties of the semiconductor and thus shift its activation wavelength to the visible part of the spectrum [13].

In this paper, we did not consider any machine learning (ML) or optimization technique, but some authors formulated a database containing the elemental composition (C, H, O, N, O, S, and other atoms) of the feedstock and the ash content in order to create a model for the prediction of the biochar properties. In this context, they used statistical methods such as Regression Modelling and Multiple Linear Regression to approximate the properties of biochar products [14,15].

Our study provides an overview of the latest achievements related to the development of biochar and biochar-based composites, which have great potential for solving complex problems in environmental protection. Biochar sources, techniques for the preparation of biochar and biochar-based photocatalytic composites, physico-chemical properties of such materials, mechanisms of pollutant degradation, and applications of these materials for the removal of antibiotics and dyes are described in detail. The final part of this paper briefly outlines guidelines for future research in this area.

2. Biochar

2.1. Biochar Preparation

Biochar (BC) is a sustainable, low-cost, stable, and environmentally benign material that is mainly produced by thermochemical processes from biomass originating from plants or animals. Biochar is a typical pyrolytic solid product of biomass whose molar ratio of hydrogen to organic carbon is less than 0.7, and the molar ratio of oxygen to carbon is less than 0.4 [16].

Thermochemical processes which can be used to treat biomass are varied, including pyrolysis, hydrothermal carbonization, torrefaction, and gasification [17,18,19]. Each of these processes is carried out under specific operating conditions and feedstock requirements, which ultimately affects the quality of the desired product.

In general, based on the target products, biomass valorization processes can be divided into two categories: (a) processes whose purpose is to produce gaseous bio-energy products such as bio-oil and synthesis gas (or syngas) with biochar as a by-product and (b) processes where biochar is the main product and syngas containing hydrogen, carbon monoxide, methane, etc., is a by-product which can be directly combusted to generate electricity or heat. The syngas produced by thermochemical conversion of biomass is a mixture of H₂, CO, CO₂, and CH₄. This syngas can be used as a combustion fuel in a combined heating and power system, or it can be converted into high-value products such as methanol and dimethyl ether. Obviously, thermochemical processes enable the conversion of biomass into solid residue (biochar), condensable liquid (bio-oil), and non-condensable gaseous product (syngas) (Figure 1).

This study focuses on the production of biochar as the main product, which can serve as an adsorbent or as carrier of photocatalysts for various applications. It is important to note that the choice of the appropriate process for the production of biochar strongly depends on the physico-chemical properties of the biomass used, which will also affect the operating conditions, e.g., heating rate, residence time, maximum temperature, and other factors. As will be shown below, the most suitable thermochemical processes for the preparation of biochar as the main solid product are pyrolysis and, to a lesser extent, hydrothermal carbonization, while gasification and dry torrefaction are of less importance [17].

2.1.1. Pyrolysis

The most important physico-chemical properties for the development of heterogeneous photocatalysts, which mostly include a large specific surface area, high porosity, and suitable functional groups, can be achieved by pyrolysis. Pyrolysis (sometimes referred to as the devolatilization process) can be defined as a process that thermally decomposes biomass by its heating at elevated temperatures under controlled inert conditions (in the absence of oxygen or under an inert gas atmosphere such as nitrogen) [17,20]. According to operating conditions, especially heating rate, pyrolysis can be divided into slow pyrolysis and fast (flash) pyrolysis; although, an additional category of microwave-assisted pyrolysis can also be found in the more recent literature [19].

In practice, slow pyrolysis (also called conventional carbonization) is mostly preferred to prepare biochar as the main product because it gives the highest solid product yield of 35–50 wt.%. Slow pyrolysis occurs at 300–800 °C, with heating rates of 5–10 °C min⁻¹ and at a relatively long residence time of more than one hour [18]. The biochar yield is maximized during slow pyrolysis due to secondary reactions of polymerization, which are accomplished by extended vapor residence time.

During fast pyrolysis in the temperature range 400–600 °C, the heating rate of biomass is much higher (300–800 °C min⁻¹), but the vapor residence times are much lower (0.5–10 s). Due to the higher heating rates and lower residence time, fast pyrolysis produces a higher proportion of bio-oils and a lower proportion of biochar (15–35 wt.%) [19]. Flash pyrolysis (sometimes called very fast pyrolysis) is characterized by very fast heating rates (>1000 °C/s), high reaction temperatures (900–1300 °C), and short residence time (<0.5 s). During this process, almost identical products are produced as during fast pyrolysis, but smaller biomass feed particles (<0.2 mm) are required for the successful operation of the process.

The key advantage of microwave-assisted pyrolysis is microwave heating. Microwave heating enables the selective activation of the solid oxygen-containing components of the biomass, reducing the temperature of decomposition compared with conventional pyrolysis. In addition, microwave pyrolysis is faster and more effective, resulting in energy savings and shorter processing times. Although it is not a widespread method, preparation of biochar using the sustainable microwave pyrolytic approach has some advantages over conventional pyrolysis, including high biochar yield, low energy requirement, excellent textural characteristics, and good adsorption efficiency [21]. The resulting biochar has a higher surface area (up to 450–800 m²/g) and more functional groups than conventional biochar [22].

Contrary to pyrolysis in an inert gas atmosphere, there have been a small number of studies investigating the product distribution and composition in the presence of other gases (such as CO, CO₂, CH₄, and H₂) as well as steam. Pyrolysis under a steam atmosphere (or steam pyrolysis) has been scarcely studied in the literature [23]. It is known that the properties of biochar depend on several factors, such as the type of feedstock (chemical composition, structure, particle size, and moisture content), the choice of the appropriate hydrothermal conversion process and process conditions (temperature, pressure, oxidative conditions, etc.), pre-treatment (drying, pre-treatment with distilled water, and acid or alkaline treatment), and post-treatment procedures (various forms of activation and functionalization). The key features of photocatalysts on which their efficiency depends are surface functional groups and textural features such as surface area and porous structure (pore volume and pore size distribution). In general, the surface area of biochar can be increased in different ways, e.g., before, during, and after pyrolysis. Before pyrolysis, this can be achieved by various methods of impregnation, mixing with certain minerals, and by acid (H₃PO₄, H₂SO₄, and HCl) or alkaline pre-treatment (e.g., KOH and NaOH). During pyrolysis, the specific surface area of the biochar can be increased by steam treatment, which contributes to partial devolatilization and the formation of crystalline carbon in the biochar or the removal of products of incomplete combustion. The same effect can be achieved by CO₂ purging during pyrolysis due to the formation of more pores. After pyrolysis, the specific surface area of the biochar can also be increased by treatment with HCl, which removes inorganic fractions and the soluble organic carbon associated with these inorganic fractions. The additional effect of acid or alkaline treatment is associated with pH neutralization, depending on the expected application.

Sewage sludge is a non-lignocellulosic biomass. This is a complex mixture of water, microorganisms, undigested organic matter (proteins, peptides, lipids, and polysaccharides) and inorganic substances (nitrogen, phosphorus, carbonates, phosphates, sulfates and nitrates, and heavy metals), and other elements. The preparation of sludge carbon consists of two processes: (a) drying and (b) activation, which can be physical or chemical. The physical activation of sludge consists of pre-carbonization that takes place in the temperature range of 400–700 °C and an activation process (pyrolysis) with gas or steam at a high temperature. The advantage of steam as an activating gas is the creation of a wide pore size distribution [24]. If sludge or previously carbonized sludge is mixed with some reagent (acid, base, or salt) and subjected to high temperature in an inert atmosphere, this process is called chemical activation. By pyrolysis of sewage sludge, biochar is obtained with high carbon content. Biochar prepared by the process of steam pyrolysis of sludge with and without hydrochloric acid (HCl) in the temperature range of 500–900 °C has been accomplished by Sierra et al., 2024 [25]. The advantage of treatment with acid after pyrolysis is the unblocking of the pores, which is a consequence of the increase in the BET surface area, while the disadvantage in comparison with the previous acid treatment is the lower yield and loss of the magnetic properties of biochar. The advantage of acid activation in the temperature range of 500–900 °C is the formation of magnetite, which facilitates the recycling of biochar from sludge in wastewater treatment, as well as an increase in microporosity. The samples prepared by pyrolysis with steam (SCS series) have shown basic properties within the pH range of 7.8 to 9.1. The increase in pH with the increase in activation temperature is explained by the thermal decomposition or desorption of acidic functional groups, mainly C=O and -OH or -NH, as confirmed by FTIR analysis.

2.1.2. Hydrothermal Carbonization

Hydrothermal carbonization or wet pyrolysis is a promising thermochemical conversion process that uses heat to convert wet biomass feedstocks into a carbon-rich hydrochar without the need for an energy-intensive pre-drying [26]. Hydrochar produced by a hydrothermal process is a similar product to biochar. However, contrary to the biochar which is produced as a solid product in a dry carbonization process like pyrolysis, hydrochar is produced as a slurry (a two-phase mixture of solid and liquid). Thus, biochar and hydrochar significantly differ from each other in terms of their physical and chemical properties [27]. Different feedstocks, including aquatic biomass, agricultural residues, and industrial and animal wastes, can be used for hydrothermal carbonization. Generally, hydrothermal technologies can be categorized into three groups (carbonization, liquefaction, and gasification) based on their operating conditions [28].

Hydrothermal carbonization is a low-temperature process, which is usually carried out below water critical temperatures (between 180 and 250 °C) in a lab or bench-scale batch reactor under autogenous (automatically generated) pressure, and a residence time that can vary from minutes to several hours [29]. Compared to pyrolysis, hydrothermal carbonization is performed at a lower temperature due to the availability of hot water during the reaction. Therefore, the carbon conversion is lower than in pyrolysis, resulting in higher atomic H/C and O/C ratios. Obviously, hydrochar has higher atomic H/C and O/C ratios compared to those in biochar. Contrary to hydrothermal carbonization, which is used to produce hydrochar as the main solid product, hydrothermal liquefaction and hydrothermal gasification are used to produce hydrochar as a by-product or co-product in addition to bio-crude oil and syngas [30]. It is important to emphasize that the further development of industrial hydrothermal carbonization plants depends primarily on the design of continuous reactors that allow a continuous supply of water.

2.1.3. Gasification

Biochar can be also prepared by gasification. This high-temperature (750–1000 °C) thermochemical process can be performed at small residence times and in a partly oxidizing environment created by adding air, oxygen, steam, or other oxidizing agents. The main product of this process is syngas, and the biochar yield is very small (5–10%) [31]. The resulting gas mixture is usually combusted to generate power. It is interesting to note that incomplete combustion (when the oxygen concentration is not sufficient) can also lead to the formation of biochar, but in very small quantities (1.5–2 wt.%). Gasification is, therefore, not considered as a suitable technology for the preparation of biochar.

2.1.4. Dry Torrefaction

Dry torrefaction or mild pyrolysis occurs at temperatures from 200 to 300 °C, a small heating rate (10–15 °C s⁻¹), a residence time from 30 min to 4 h, in the absence of air, and at atmospheric pressure. This is a pre-treatment procedure usually used to remove moisture from biomass and densify it, hence increasing the heating value of the biomass, improving the biomass’s physical and chemical properties and leading to high C/H and C/O ratios, etc. Torrefaction usually produces a solid mass yield of 70–80 wt.% [32]. Typical raw materials for the torrefaction process are the wastes from forestry and agriculture. Torrefaction of biomass usually results in a high-grade biofuel, which can be used as a replacement for coal in electricity and heat production.

It has been found that the surface area, micro pore area, and total pore volume increase significantly, along with the biochar quality and energy yield as a result of the pre-treatment of the feedstock (vacuum freeze drying, pelleting, drying, and ultrasonic assisted-vibration pelleting) [33].

According to research [34], the interaction between the mill balls and raw biomass promotes the cleavage of chemical bonds and the surface morphology changes, and chemical functional groups are changed. A previous study [35] has found that the ball-milling process increases surface area, enriches the oxygen-containing functional group, and reduces particle size.

Table 1. Biochar production processes and biochar yield.

Thermochemical Process	BC Yields, wt.%	Pyrolysis Temperature, °C	Limitations/ Residence Time	Ref.
Pyrolysis slow fast very fast	higher (35–50) lower (15–35)	300–900 300–900 900–1300	absence of O2, >1 h, and 0.5–10 s <0.5 s	[18] [19]
Gasification	very low (5–10)	>700	in the presence of air, O2, or steam	[31]
Hydrothermal carbonization	carbon content of the biochar is greater than that of the pyrolysis	<250	from minutes to several hours	[29]
Dry torrefaction	high (70–80)	200 to 300	absence of air; 30 min–4 h	[32]

provides an overview of the biochar preparation process, biochar yields, and production conditions.

2.2. Biochar Sources

As already mentioned, the choice of biochar source/feedstock affects the working conditions during thermochemical conversion, including the economics of the process, as well as the quality of the desired product. Based on the initial moisture content of the biomass, the feedstock for biochar production can be classified as dry or wet (Figure 1). Dry biomass like some agricultural residues have low moisture content (<30%), while wet biomass such as agriculture and animal wastes, algae, sewage sludge, etc., have higher moisture content (>30%). In this context, it should be noted that wet raw materials should be dried before processing, which can be an energy-intensive approach and reduce the overall efficiency.

Biomass originating from plants mainly contains polysaccharides, namely starch (amylose and amylopectin) and lignocellulose (lignin, hemicellulose, and cellulose) which have a higher specific surface area and require a lower decomposition temperature than solid waste and animal litter. The reaction of lignin decomposition is accompanied by the release of gaseous hydrogen and methane and the condensation of aromatics, which leads to an increase in the porosity of biochar [36,37].

Biomass of animal origin contains polysaccharides such as alginates isolated from brown algae (Phaeophyta), chitin, which is predominantly found in the cell walls of fungi and the exoskeletons of crustaceans and insects [38], and glycogen, which is the primary storage form of carbohydrates in vertebrates.

2.2.1. Agriculture and Food Waste

Various agricultural and forest residues have significant potential as biochar raw materials due to their high cellulose content. The main sources of biochar are straw, husks, grass, walnut shells, coconut palms, fruit peels, etc. [39]. Food waste represents the largest type of waste in the world and accounts for 50% of municipal solid waste [33]. Moyo et al. [40] reported the use of maize stover (MS) and rice husks (RHs) for the production of biochar by the pyrolytic process. Biochar prepared in this way was effective in xenobiotic removal. The annual global production of corn is about 1 billion tons, and stover accounts for about half of its weight [41]. On the other hand, global rice production is about 700 million tons, and rice husks make up about 20% of the total rice mass [42]. The yield of biochar from rice husks depends on the pyrolysis temperature, and according to [43], it is about 38% percent. The share of organic carbon for RH biochar is between 30 and 50% and again depends on the pyrolysis conditions, while for MS biochar, it is 45–64%.

A huge amount of rice straw as a raw material for the production of biochar is present, 731 million tons globally, of which 90% is in Asia, followed by cabbage in South Korea at 3 million tons (30% as waste) and wood chips at 2.4 million tons (in the form of furniture waste) [44].

For the production of pyrolytic biochar, Hoslett et al. [45] heated mixed food residues and plant residues at 300 °C for 12 h in an innovative heat pipe reactor. Carbon and oxygen are the main elements that make up biochar with the presence of traces of minerals with the highest content of potassium. In addition to them, biochar contained aromatic sp2 groups, i.e., π electron donors. Biochar prepared in this way was used for tetracycline adsorption with initial concentrations of 20 mg/L and 100 mg/L, whose adsorption capacity was 2.98 mg/g and 8.23 mg/g, respectively. The mechanism of tetracycline adsorption took place through π-π electron–acceptor–donor interactions [46].

In the research carried out by Fatimah et al. [47], the authors prepared by direct pyrolysis a magnetic nanocomposite photocatalyst based on palm leaves biochar impregnated with a nickel (II) chloride precursor. The composite has shown high photocatalytic activity with complete methyl violet removal in 30 min.

2.2.2. Animal and Poultry Manure Wastes

Fertilizer and sewage sludge are characterized by a high content of nutrients and can be used to produce biochar with a higher nitrogen content compared to biochar produced from sugar cane and eucalyptus waste. For example, the nitrogen content in biochar produced from different raw materials decreased as follows: sewage sludge 31.7 g/kg > sugar cane > 14 g/kg and eucalyptus 4 g/kg. A lower concentration of nitrogen was also recorded with an increase in pyrolysis temperature [48]. Fertilizers mainly contain cellulose, hemicellulose, lignin, proteins, lipids and fats, and a considerable amount of nutrients. For example, cattle manure contains 14–35% cellulose, 11–32% hemicellulose, 13–15% lignin, 8–30% protein, and 15–32% lipid and fat [49]. During pyrolysis, first, hemicellulose is gradually decomposed, and then, with increasing temperature, the cellulose is decomposed, in the temperature ranges of 220–315 °C and 315–400 °C, respectively [50], and lignin degradation occurs in a wide temperature range of 100–900 °C [51]. The yield of biochar produced by the pyrolytic process of fertilizer is from 40% to 60% and depends on the type of fertilizer and pyrolysis conditions, while these values for wood are from 20% to 30% and depend on the temperature of pyrolysis [52].

Research [53] has shown that the production of biochar from poultry litter (PL) can be used as an organic fertilizer rich in nutrients to improve the nutritional status of the soil. After pyrolysis of PL, poultry biochar (PLB) was produced with significantly higher concentrations of organic carbon, available nitrogen, phosphorus, potassium, calcium, total phosphorus, potassium, calcium, magnesium, and iron, which had a positive effect on increasing plant growth and biomass production.

2.2.3. Industrial Wastes

Activated sludge has proven to be very efficient in removing organic impurities. However, the sewage sludge produced after the wastewater treatment process with activated sludge can contain viruses, pathogens, and heavy metals. One of the ways to dispose of such harmful sewage sludge is the pyrolysis process, which can be used to obtain biochar, biofuel, or bio-oil [54]. Biochar obtained from sewage sludge contains high amounts of ash [55].

2.2.4. Algae and Crustacean’s Wastes

In general, biochar from macroalgal sources has a smaller surface area than biochar from lignocellulosic sources [56]. Recently, Mondal et al., 2024 [57] prepared biochar from five types of macroalgae using a pyrolysis process conducted at 600 °C for 40 min under nitrogen flow at 4 L min⁻¹ for 20 min before heating, during and after the pyrolysis process, until the temperature decreased below 100 °C to prevent ignition inside the furnace. The physico-chemical properties of biochar from macroalgae show a disordered structure and high porosity, as well as the presence of a large number of functional groups containing oxygen and inorganic minerals such as Ca, Na, Mg, K, and P and insignificant amounts of heavy metals (Cd, Cu, Cr, Pb, As, Zn, and Hg). These microalgae biochar were able to remove 95–100% of methylene blue by the adsorption process. The high pH value varied between 9.86 and 10.95, depending on the type of macroalgae, but this high pH value suggests its possible application in agriculture for acidic soil remediation.

Other reported works also prepared biochar (BC) based on macroalgae using the pyrolysis method; they are first washed in their raw state with ultrapure water and then subjected to drying at 120 °C for 24 h. After that, they are ground and pyrolyzed at 650 °C for 2 h in a muffle furnace to obtain BC [58]. Using nitrogen gas to purge the pyrolytic fumes, the contaminants were eliminated from BC by washing them with ultrapure water. Following a 24 h oven dry cycle at 80 °C, BC was ground and sieved to produce particles that were roughly 125 μm in size, ready for use in additional studies [59].

2.3. Biochar Structure and Physico-Chemical Properties

Among the most important features on which the application of biochar in catalytic processes depends are the common physico-chemical properties, especially porosity, specific surface area, elemental composition, and presence of surface functional groups. In addition to carbon (C) as the main element present in the structure of the biochar and hydrochar, other constituents are hydrogen (H) and oxygen (O) as well as a small amount of nitrogen (N) and sulfur (S). In addition to organic elements, biochar contains some inorganic elements like Ca, Mg, P, Si, Al, Fe, and K as well as a trace of metals (Cu, Ni, Zn, Co, and Mn). The physico-chemical properties of biochar are highly associated with the raw materials (chemical composition, contents of moisture, and particle size) used to produce the biochar, the type of process, and the key process conditions, such as temperature, heating rate, vapor or solid residence time, and reaction environment. In comparison with most conventional activated carbon, biochar possesses a well-developed mesoporous structure and abundant surface functional groups. Therefore, it can be used as an adsorbent and versatile catalyst as well as a catalyst support.

The porosity of biochar depends on the structure of the parent material. The composition of parent ash will be almost identical for the milder temperature conditions (400–600 °C) of slow pyrolysis and gasification, while for higher temperatures (up to 850 °C), alkaline elements such as sodium and potassium may evaporate to a certain extent while other elements remain almost completely in the biochar [60]. Samples prepared only by steam activation showed a trend of increasing specific surface area (S_BET) with increasing temperature, i.e., the highest value of 113 m²/g was achieved at a temperature of 900 °C. The decrease in the average pore size with increasing temperature led to an increase in the specific surface area in pores up to 100 Å (micro- and small mesopores), while a decrease in the volume of large mesopores and macropores was recorded [25].

Acid washing after steam activation of samples (SCS-A-800) at a temperature of 800 °C showed a significant improvement in the porous microstructure, more precisely, a threefold increase in S_BET, at 323 m²/g, with a total pore volume of 0.553 cm³/g. Furthermore, the total micropore value of SCS-A-800 is four-times higher than for the steam activation only (SCS) sample and amounts to 103 m²/g [25]. Thus, the acid washing works well to reveal the pyrolysis-generated pores that were masked by pore occlusion. A portion of the material obstructing the porous structure is removed by the acid washing, improving pore accessibility. To improve the outcomes of pyrolysis with steam, it is therefore highly recommended to perform an acid wash using HCl. It is important to consider that the removal of inorganic materials, which are essentially non-porous, may be the cause of the increase in specific surface area.

Li et al., 2020 [61] studied the effect of low-alkali–hydrothermal treatment on the TiO₂ structure and photocatalytic performance. Alkali ions combine with oxygen to form alkali metal oxides on the TiO₂ surface, or they penetrate into the TiO₂ lattice and limit the growth of TiO₂ nanocrystals. The resolute factor which affects the surface structure and catalytic properties of the photocatalyst is particle size and shape. They found that the surface area of alkali–hydrothermally treated TiO₂ was 6-times higher and pore volume was 10-times higher than the pristine TiO₂. Wang et al., 2020 [62] also reported that TiO₂-alkaline biochar (Ti-KBC) showed the smallest bandgap and the best photocatalytic properties, as well as has the highest specific surface area, compared to TiO₂-biochar (Ti-BC) and TiO₂-ironized biochar (Ti-FBC). In a weak acidic medium, at pH = 5, the degradation efficiency of enrofloxacin was 85.25%, and the main role in the mechanism of the reaction was ascribed to the superoxide anion radical. The material is robust even after five repeated cycles of photocatalytic degradation of enrofloxacin.

3. Preparation of Biochar-Based Photocatalytic Composites

The great advantage of biochar over typical catalyst supports is that it can be easily produced from renewable sources, which is particularly important for a carbon-neutral economy. Biochar has proven to be an effective catalytic support due to its high specific surface area, high porosity and stability, and abundant surface functional groups (e.g., -OH, -COOH, -NH, etc.) that enable its additional functionalization and improvement of its physico-chemical properties depending on the process requirements, as well as acceptable optical properties (high absorption in the visible and near-infrared regions of the electromagnetic spectrum) [63].

A detailed characterization (FTIR, XPS, etc.) was reported by many authors to demonstrate the specific nature and abundance of these groups across the different biochar samples. In this context, Yaashika et al. (2020) gave a comprehensive overview of the various instrumental analytical techniques used for the characterization of activated carbon and biochar [64].

Desirable properties of biochar are the prevention of the agglomeration of nanoparticles and improvement of their reactivity. Biochar-based photocatalytic composites offer a promising solution for the removal of organic pollutants from wastewater through a combination of the desirable adsorption and physico-chemical properties of biochar and the unique properties of nanomaterials, like TiO₂, ZnO, MgO, g-C₃N₄, and others.

A composite material is generally defined as a combination of two or more chemically different materials with a clearly defined boundary between the components and with properties that are better than those of the individual components. Therefore, the preparation of biochar-based photocatalytic composites is not an easy task and is mostly based on nanomaterials engineering. In general, such composite materials can be produced by two approaches: (a) by pretreatment of biomass or post-treatment of the produced biochar with a suitable metal precursor, magnetic components, functional materials, etc., and (b) by integration of semiconductor materials and biochar directly during the corresponding synthesis route [65,66]. The choice of the appropriate preparation procedure depends primarily on the area of application and the expected properties of the final product. The typical methods used for the preparation of biochar-based photocatalysts include sol–gel, ultrasound-assisted synthesis, thermal polycondensation, and solvothermal and hydrothermal synthesis.

3.1. Sol–Gel Synthesis

The preparation of biochar-based photocatalytic composites using the sol–gel method takes place in three steps. The first step involves the thermal decomposition of biomass to create biochar templates. The second step involves treating the biochar with acid to raise surface oxides and lower its pH, depositing catalytic nanoparticles on its surface. The third step involves the calcination of the obtained catalytic nanoparticles loaded with biochar in order to obtain a stable structure [12].

Biochar/TiO₂ composite was prepared by Kim and Kan [67] by using the sol–gel method. They used biochar prepared by flash carbonization of corn cobs at 873 K with a specific surface area of 134 m²/g. For the chemical activation of biochar, they used nitric acid with a pH value of 3 to increase acidic surface oxides. Then, the acid-treated biochar was washed with deionized water several times and dried at 70 °C for 24 h. For the preparation of TiO₂/biochar, acid-treated biochar was dispersed in a solution with a 1:3 ratio, with 97% Titanium isopropoxide as a precursor and ethanol, stirred for 1 h, and then, a solution containing HCl and ethanol was added and stirred for another hour. Then, the sample was vacuum filtered, dried for 24 h at 100 °C, crushed, and calcined for one hour at 325 °C. Using this process, they produced biochar/TiO₂ with a specific surface area of 383 m²/g. Photoactive TiO₂ nanopowder has a specific surface area in the range of 35–65 m²/g, and it was used to compare the photocatalytic activity with a prepared composite for sulfamethoxazole (SMX) antibiotic degradation in water. Hydrophobic interactions between biochar and SMX were responsible for greater adsorption of antibiotics on the biochar/TiO₂ composite. When compared to the biochar/supported TiO₂, the raw biochar exhibited more SMX adsorption because it had more carbon sites for the π-π interaction needed for SMX adsorption. Also, this research revealed that the addition of a very small amount of bicarbonate enhances antibiotic removal and its mineralization, while the presence of nitrate only slightly improves the removal efficiency, respectively.

3.2. Ultrasound-Assisted Synthesis

From biomass such as starch, Clark et al. have developed a highly mesoporous carbon material (V_meso > 0.3 cm³ g⁻¹) with a high specific surface area (S_BET > 500 m² g⁻¹) under the trademark “STARBON^®” [68]. In addition, biochar has been developed by thermochemical treatment at temperatures of 350–700 °C in a reactor with a limited amount of oxygen. These materials are characterized by the presence of surface functional groups, which makes them ideal for the preparation of new inorganic–organic photocatalytic composites. Sonication ensures that catalyst preparation can be carried out faster, with the use of mild and green synthesis conditions, because cavitation phenomena are created, i.e., local temperature overheating points of around 5000 °C, and high pressures of around 1000 bar and rapid heating/cooling rates of around 1000 °C/s are produced. Ultrasound can also control the speed of the crystallization process as well as the crystal size distribution and can reduce particle agglomeration [69]. Procedures for the preparation of composite materials based on ultrasound or a combination of ultrasound and conventional methods (e.g., impregnation, sol–gel, precipitation, etc.) are increasingly described in the literature. For example, Lisowski et al. (2017) [70] prepared hybrid TiO₂-based biochar materials by ultrasound-assisted wet impregnation. It was found that ultrasound acts as an “interfacial mediator”, i.e., it promotes the formation of intimate interfacial contact between biochar and TiO₂. This leads to an improved transfer of photogenerated charge carriers and prolongs the stability of the charge carriers due to a slower recombination rate. In [71], Lisowski and coworkers describe the preparation of crystalline TiO₂ on lignocellulosic carbon materials using a low-temperature ultrasonic-assisted sol–gel method coupled with citric acid as a crosslinking agent. The main advantages of such an approach are milder and environmentally friendly reaction conditions, as well as the elimination of the high-temperature treatment (calcination) required by the conventional sol–gel method.

3.3. Thermal Polycondensation

Biochar-modified carbon nitride (BC/CN) composites were produced via a thermal polycondensation process in a muffle furnace [72]. Melamine as a precursor together with crab shell (BC) with a certain mass ratio were put in a ball-mill machine at 360 r min⁻¹ for an hour and were then put into a muffle furnace at 550 °C for two hours. After those steps, the final product was ground again in a ball mill at 360 r min⁻¹ for 1 h. To produce pure CN, only melamine was used.

3.4. Solvothermal Synthesis

Mao et al. [73] synthesized by the solvothermal method a new Bi₂WO₆/NSBC composite from Bi₂WO₆-loaded N and S co-doped corn straw biochar, and it was used for the removal of ciprofloxacin (CIP) under visible-light irradiation. To prepare NSBC, straw was impregnated with a different concentration of sulfourea solution prepared in deionized water, then dried for 2 days at 70 °C. This was followed by a pyrolysis process in a nitrogen atmosphere at 550 °C for 2 h followed by grinding. Three types of NSBC composites were synthesized with three different mass ratios of thiourea and corn straw. For the preparation of Bi₂WO₆/NSBC by solvothermal reaction, a 1:1 molar ratio of sodium oleate and Bi(NO₃)₃ × 5H₂O was added to 40 mL of ethylene glycol, while a half as small amount of Na₂WO₄ × 2H₂O was dissolved separately in ethylene glycol with heating. The two solutions are then mixed for 1 h at room temperature. After that, NCBC is ultrasonically dispersed in ethylene glycol for half an hour, then added to the solution mixture and mixed for 2 h. Then, the mixture is transferred to a Teflon-lined autoclave and heated to 180 °C for 20 h and afterwards cooled, washed several times with deionized water and ethanol, dried at 70 °C for 12 h, and crushed. Finally, the obtained material is Bi₂WO₆/NSBC. The composite prepared in this way has exhibited a removal efficiency of CIP of ~90.33% in 75 min and can be used in a wide pH range from 3.0 to 9.0. In order to prevent Bi₂WO₆ agglomeration, N and S co-doping of the BC results in densely interconnected fibrous structures, strong catalytic properties, and advantageous specific surface areas. In this work as well, the synergistic effect between photoactive species and biochar results in a decrease in the bandgap energy and a shift of the absorption maximum to the visible part of the spectrum, as well as a decrease in the possibility of recombination of photogenerated pairs of electrons and holes which results in increased photocatalytic activity.

3.5. Hydrothermal Synthesis

A series of TiO₂/biochar composite catalysts were prepared by hydrothermal synthesis. Walnut shells were used as biomass for the production of biochar. Biochar was prepared by a slow pyrolysis process under a nitrogen atmosphere at 500–800 °C for 2 h in a tubular furnace. An increase in the calcination temperature from 500 to 700 °C resulted in an increase in surface area by as much as 27.4 times, so that at 700 °C the surface area was 66.06 m²/g, while the pore volume increased by 4.4 times and at 700 °C was 0.066 cm³/g. An increase in the calcination temperature by another 100 °C resulted in the collapse of the pore structure and, consequently, a decrease in the pore volume by 2.5 times, and at 800 °C, the pore volume was 0.026 cm³/g; the surface area also decreased by 3.7 times and at 800 °C was 17.74 m²/g [74]. For the preparation of the composite material, the direct hydrolysis method reported by Makrigianni et al. (2015) was used [75]. Tetrabutylorthotitanate (99.8%) was added dropwise to 100 mL of ultrapure water and stirred for 12 h. After that, a certain quantity of biochar was added, and the suspension was stirred for 12 h. The suspension was dried at 80 °C for one day and calcined at 500 °C for 1 h under an N₂ atmosphere. Furthermore, the identical method was used to prepare blank TiO₂ without the inclusion of biochar. Composite materials prepared in this way showed higher photocatalytic activity than pure TiO₂. The composite prepared with the 0.2:1 biochar–Ti ratio was labeled as CT0.2/1 and it showed the highest photocatalytic activity towards methyl orange (MO) degradation with a decolorization rate of 96.88% and a mineralization efficiency of 83.23%, but also exhibited the possibility of recycling for at least five repeated cycles with a slight decrease in photocatalytic activity [74].

4. Advantages of Biochar in Photocatalytic Degradation of Pollutants

Photocatalytic composites based on biochar show high efficiency in removing various organic pollutants from water and wastewater. This is due to the specific advantages of such materials, which include the ability of biochar to support semiconductor nanoparticles, a large specific surface area and a huge number of active centers, the reduced bandgap, the ability of biochar to act as an electron reservoir, and the improved charge separation/avoidance of rapid recombination of the photogenerated electron–hole pairs. A schematic representation of the advantages of biochar in the photodegradation of pollutants is shown in Figure 2.

4.1. Supporting Nanoparticles

When it comes to the influence of the addition of a photocatalyst on the activity of the composite, the dosage of the photoactive species, i.e., the ratio of the photoactive species and the biochar, must be taken into account. The results of the research [76] showed that a lower loading of photoactive species compared to biochar gives better removal results, which are explained by increased electron transport due to the presence of a higher concentration of phenolic groups on the biochar. Namely, it must be examined what is the optimal concentration ratio of photoactive species and biochar, because, otherwise, an excess of photoactive species will reduce the catalytic activity of the composite due to the phase change and the inability of photons to reach the photoactive surface.

4.2. Increasing Surface Area and Active Sites

The catalytic activity of biochar originates from the presence of the oxygenated functional groups -OH, -COOH, and -COOR on its surface. In addition, the presence of nitrogen, phosphorus, potassium, magnesium, and calcium in biochar can act as catalysts or adsorbents. Active sites on biochar can help the even distribution of semiconductor photocatalysts either by a chemical modification process or impregnation and produce a composite with increased photocatalytic activity, but at the same time, they reduce the aggregation of the photocatalytic component and facilitate the catalyst recycling process.

In a paper published by Lazarotto et al., 2020 [77], it is evident that the insertion of the semiconductor photocatalyst TiO₂ into the biochar significantly increases the surface area of the composite up to 3 times compared to the biochar itself, and in this way, the mass transfer resistance for antibiotics to react with active sites of biochar is prevented.

Contrary to this result, Peng et al., 2019 [78] made a BC/TiO₂-CuO composite, where the incorporation of TiO₂ caused a drop in the specific surface area and an increase in the pore size from about 2 nm (biochar) to about 14.5 nm, because it is assumed that the structure of the biomass was damaged during pyrolysis, and that the TiO₂ led to pore blocking.

4.3. Narrowing Bandgap

The narrowing of the energy band in the biochar–semiconductor photocatalyst composites is due to the creation of an electron capture site localized under the conduction band of the semiconductor, thus preventing recombination, as well as the created metal–oxygen–carbon bonds between the biochar and the photoactive species, which facilitates charge transfer. According to the research (Umrao et al., 2014) [79], the created Ti-O-C covalent bond results in a decrease in the bandgap energy from 3.2 to 2.63 eV. In addition, Lazarotto et al., 2020 [77] reported that the bandgap energy of the composite is reduced by 1.95 times (1.75 eV) compared to pure TiO₂. This metal–oxygen–carbon covalent bond is formed during the synthesis process by substitution of the oxygen atom in the TiO₂ crystal lattice with the carbon from biochar. Thus, in more detail, the covalent bond is responsible for increasing the photocatalytic activity, because narrowing the energy gap actually enables a faster transfer of photogenerated electrons from biochar to the conduction band of the semiconductor and the holes from valent band of the semiconductor and biochar matrix.

4.4. Electron Reservoir (Sink)

The ability of biochar to mediate redox reactions is determined by the number of electrons that can be stored in biochar, and the quinone parts that are produced by the thermal reaction during the pyrolysis are directly responsible for this. Phenolic and hydroxide groups are responsible for the electron donating capacity of biochar produced at low temperatures, while conjugated double bonds or π-electrons are responsible for the electron donating capacity at higher temperatures [80].

4.5. Enhanced Charge Separation/Prevention Recombination

Since biochar also absorbs light energy, it injects photo-excited electrons into the photoactive TiO₂ species, creating an energy difference between the biochar in the middle gap above the TiO₂ valance band, thus preventing charge recombination, but also acting as an electron-trap. After photoexcitation of the composite, the photogenerated electrons from TiO₂ can either reduce the oxygen molecule O₂ to the radical anion O₂^∙− or directly react with a pollutant molecule (dye or antibiotic), while h⁺ can react with a water molecule and create an ∙OH radical, as well as directly react with a pollutant molecule.

5. Photodegradation Mechanism of Biochar-Supported Photocatalyst

5.1. Adsorption

This work focuses primarily on the photocatalytic degradation of selected pollutants. However, it is necessary to point out the role of adsorption in such systems. Although adsorption is a simple separation process, it has the decisive disadvantage that it only enables the transfer of pollutants from one phase to another (recovery process). Photodegradation, on the other hand, is a destructive process that eliminates the need for additional processing of the relevant photocatalytic materials after they have been used. In practice, integrated technologies that enable simultaneous adsorption and decomposition play an important role, as adsorption causes an increased concentration of reactants on the surface of the solid material and enhances the interaction of reactants with short-lived free radicals [81].

The adsorption process requires highly porous materials (adsorbents) with good selectivity for a specific pollutant. One of the adsorbents that can be successfully used for the adsorption of dyes and antibiotics is biochar. The pollutant can be adsorbed to the surface of the adsorbent in two ways: (a) by means of van der Waals forces and hydrogen bonds (physical adsorption) or (b) by a chemical bond to a specific functional group (chemisorption) [82].

Ahmed et al., 2017 [83] studied the sorption capacity of different antibiotics on functionalized biochar and found that the distribution coefficient was ranked as sulfathiazole > sulfamethoxazole > sulfamethazine. They found that sorption strictly depends on the pH of the environment, and that the sorption of neutral sulfonamide species occurred mainly through π⁺-π electron–donor–acceptor (EDA) and Lewis acid–base interactions, while sorption of negative species occurred mainly through proton exchange with water, forming a negative-charge-assisted hydrogen bond, followed by the neutralization of -OH groups by H⁺ released from the functionalized biochar surface, as well as π-π electron–acceptor–acceptor (EAA) interactions.

Li et al., 2017 [61] prepared a new composite of natural attapulgite and potato stem (1:5) by a pyrolytic process at 500 °C for 6 h and used it for the efficient adsorption removal of norfloxacin from aqueous solution. The maximum sorption capacity of the green cost-effective adsorbent was 5.24 mg/g, and it was almost 1.7 times higher than that of the pristine biochar. In another study, sawdust biochar doped with iron and zinc was prepared by Zhou et al., 2017 [84] and used as a highly efficient adsorbent for the removal of the antibiotic tetracycline and the heavy-metal copper II ions from single and binary waters.

5.2. Photodegradation

In general, holes (h⁺), hydroxyl radicals (∙OH), and superoxide anion radicals (·O₂⁻) are mainly considered to be responsible for the process of decomposition of organic compounds using photocatalysts [85]. To study the mechanism of methyl orange (MO) dye degradation using heterogeneous photocatalysis, active-species-trapping experiments were usually carried out, and EDTA-2Na, TBA, and BQ were used to quench holes, hydroxyl radicals, and superoxide anion radicals, respectively. With the addition of EDTA-2Na, TBA, and BQ, the decolorization efficiency was reduced, which indicates a different extent of MO degradation inhibition. The results of the MO degradation research using TiO₂/biochar composites indicate that all three active substances participated in the MO oxidation process, and the main role was played by hydroxyl radicals (∙OH). Although there are no significant differences in the mechanism of photodegradation on biochar-based photocatalysts and in the basic principles of their operation compared to common photocatalysts, the key properties of such composite materials discussed in the previous chapter enable overcoming the usual limitations of photocatalysis. A large specific surface area, a favorable porous structure, and a huge number of active centers allow them to have a high adsorption potential and a high efficiency for the photodegradation of various pollutants. These, among other attributes, stems from the aromatic carbon structure and the presence of various surface functional groups that favor the adsorption of pollutants through different types of interactions [36]. It should be noted that the aromatic carbon structure also facilitates the delocalization of electrons from the active sites to the receptors of the pollutant. This leads to a reduction in the recombination of electron–hole pairs and an improvement in the photodegradation of desired pollutants. The improvement of photocatalytic efficiency is also related to enhanced charge separation and the fact that the biochar acts as an electron reservoir. The hybridization of a semiconducting material and biochar also leads to a reduction in the bandgap energy and enables photodegradation in the visible range, which is particularly important for practical application in real systems. More detailed information on the photodegradation mechanism of biochar-supported photocatalysts can be found in the relevant literature [3,36,86].

In the proposed degradation mechanism (Figure 3), organic molecules and intermediates are first adsorbed on biochar thanks to the porous structure and the presence of functional groups containing oxygen, which facilitates the contact between organic molecule and the catalyst. When the catalyst or photoactive TiO₂ species are exposed to UV radiation with the wavelength equal or higher than the bandgap energy, E_g, an electron (e⁻) is excited from the valence band to the conduction band, leaving a positive hole (h⁺) in the valence band. Biochar can transfer electrons and act as an acceptor, thus preventing the recombination of electron–hole pairs. Then, photogenerated electrons can react with oxygen molecules to generate superoxide anion radicals, and through reactions with water (H₂O), they can form hydrogen peroxide (H₂O₂), primary oxygen radicals (·O₂⁻), and then give a hydroxyl radical, (∙OH) [87,88,89,90]. Hydroxyl radicals may be also generated by the trapping of holes by water molecules. Therefore, holes, superoxide anion radicals, and hydroxyl radicals would participate as active species in the interaction with organic molecules, and carbon dioxide is eventually formed. Therefore, the resulting (∙OH) radicals are very strong oxidizing agents with the standard redox potential of +2.8 V which leads to complete mineralization of organic molecule to CO₂ [74,91].

Hosseini-Monfared et al. [92] explained that RhB degradation can take place in two ways, namely through a photocatalytic (in a solution bulk reaction) and dye-sensitization (surface reaction) mechanism, respectively [93]. When the dye is adsorbed on the surface of TiO₂ particles, photodegradation of RhB takes place via N-demethylation [94]. The adsorbed dye on the N-doped TiO₂ surface reduces the bandgap energy from 3.0 eV to 2.6 eV and serves as a sensitizer. RhB molecules excited by visible light with a wavelength greater than 470 nm inject electrons into the conduction band of TiO₂ particles and initiate chemical degradation [95,96]. As previously mentioned, the photodecomposition of RhB in the aqueous solution is facilitated by the hydroxyl radicals (∙OH), which are created when electrons from the valence band of TiO₂ are excited to the conduction band by light irradiation. Furthermore, the excited RhB creates ∙OH and injects electrons into the TiO₂ conduction band, inducing the surface reaction. This mechanism generates hydroxyl radicals that spontaneously deethylate adsorbed RhB molecules [97] and shift the maximum absorption peak from 554 nm to 497 nm. On the other hand, there is no peak shift and only a simple drop in RhB absorbance during the solution bulk process that breaks down RhB molecules.

6. Application of Photocatalytic Composites Based on Biochar for the Removal of Organic Contaminants

6.1. Antibiotics

Photocatalytic degradation of ciprofloxacin (CIP) by rice straw biochar (RSB) and TiO₂ modified rice straw biochar (Ti-RSB) has been studied [98]. The maximum adsorption capacity for CIP at pH = 5 for RSB was 747.64 mg g⁻¹. The BET specific surface area of Ti-RSB is nearly 3-times higher than that of RSB, while decreasing of the average pore size may be due to the increase in micropores. The adsorption occurs through the C=O functional group by π-π interactions and H bond interactions. The prepared composite Ti-RSB has shown a superior removal effect on CIP degradation at pH = 5–9 through the defluorination reaction and the degradation of the piperazine ring.

Other reported works studied the removal of two different quinolone antibiotics, ciprofloxacin (CIP) and norfloxacin (NOR), from water by using an effective and recyclable Fe/Ti biochar composite (Fe/Ti-MBC) with magnetic and photocatalytic capabilities [99]. As a source of biomass for the preparation of biochar, wood chips were used, which, after drying at 105 °C, were subjected to the carbonization process at 300, 450, and 600 °C and marked as MBC300, MBC450, and MBC600. For the preparation of FeMBC300, FeMBC450, and FeMBC600, mass ratios of 2:1 wood chips and hematite were used. For the preparation of Fe/Ti-MBC300 and Fe/Ti-MBC600, Fe/Ti biochar/TiO₂ was used in a mass ratio of 2:1:1. The highest specific surface area (SSA) of raw wood chip biochar was exhibited by the one pyrolyzed at 300 °C, i.e., the surface area was 3–6-times larger than the MBC pyrolyzed at 450 and 600 °C, and the pore volume increases with the increase in pyrolysis temperature. This result is explained by the fact that as the pyrolysis temperature increases, a large number of micropores collapse, and the contact between different pores forms mesopores and macropores. The (SSA) of Fe/Ti-MBC at 600 °C is assumed to be lower than FeMBC600 due to TiO₂ loading causing pore clogging. Photocatalytic testing of the prepared composites was performed in the presence of 40 mg/L CIP and NOR for 12 h of adsorption and 6 h under visible radiation. A UV lamp, xenon lamp, and sunlight at different pH values from 3 to 9 were used as the light source. They tested tap water, sewage treatment plant wastewater, and medical wastewater. The findings demonstrated that FeMBC600 has a maximum adsorption capacity of 7.59 and 4.23 mg/g for CIP and NOR, respectively. Fe/Ti-MBC has a maximum photocatalytic degradation efficiency of 88.4% for CIP and 88.0% for NOR at pH = 6.0. They also claim that the primary adsorption processes for CIP and NOR is via the π-π interactions and polar interactions, while ⋅OH radicals and H⁺ are the main active substances for photocatalytic degradation.

Wang et al. [100] made a heterojunction nanophotocatalyst ACB-BiVO₄ based on photoactive BiVO₄ loaded onto the support ACB for the photodegradation of three types of antibiotics, namely tetracycline (TC), norfloxacin (NOR), and chloramphenicol (CAP), and the dye rhodamine B (RhB) under visible light. The optimal ratio of photoactive species to ACB support was 1:0.62, and it showed almost 6-times higher and faster efficiency than pure BiVO₄ in the degradation of RhB. The heterojunction nanophotocatalyst ACB-BiVO₄ prepared in this way completely degraded RhB and TC from surface water, while it removed 73.50%, respectively, from industrial wastewater. The OH radicals and cavities formed by photoactivation of ACB-BiVO₄ (Eg = 2.26 eV) at 545 nm are considered dominant and responsible for the mechanism of this reaction.

Xiao et al., 2023 [20] prepared a biochar/g-C₃N₄ (BC/CN) photocatalytic composite by a simple thermal condensation from melamine and waste crawfish shells. They tested this photocatalyst for degradation of enrofloxacin (ENR), which is a fluoroquinolone antibiotic used for the treatment of animals. The best results of ENR degradation in wastewater under visible light were achieved with a 1 gL⁻¹ photocatalyst prepared with the BC:CN ratio = 2%. With a degradation rate of 90% under eight hours of visible light irradiation, BC/CN-2% had the greatest photocatalytic performance among all the catalysts, while the degradation rate of pure CN was only 73.5%. When photocatalyst dosages were increased from 0.25 g/L to 1.0 g/L, there was a rapid rise in ENR degradation efficiency, which went from 63.80% to 90%. While the degradation rate of ENR increased gradually with an increase in photocatalyst dosage from 1.0 g/L to 2.0 g/L, its degradation efficiency declined noticeably. This could be because of the low transparency of the solution, which is brought on by a high concentration of catalyst particles and makes it difficult to achieve light penetration [83]. Furthermore, a high concentration of catalyst particles may interact strongly, which could lower the number of active sites accessible for photocatalytic reactions as well as the effective surface area for photon absorption.

Table 2. Biochar-supported photocatalysts for degradation of antibiotics.

Active Species/ Composite	Biomass/Pyrolysis Temp. °C/Time	Mass of Photocatalyst	Adsorption Eq. Time	Light Source/ Irradiation Time	Pollutant	Removal Efficiency %	Ref.
BC/CN-2%	Crawfish shells 550 °C/2 h	1.0 g/L	30 min	Vis light/ 8 h	(ENR)	90%	[20]
Biochar/TiO2	Flash carbonization of corn cob at 600 °C	5 g/L	30 min	UVC light 3 h	100 mL of 10 mg/L (SMX) pH = 4	75% 0.6–2.4 mg SMX per g biochar/TiO2	[67]
Biochar/ TiO2	Flash carbonization of corn cob at 600 °C	5 g/L	30 min	UVC light 6 h	100 mL of 10 mg/L (SMX) pH = 4	91%	[67]
Bi2 WO6/NSBC	Corn straw/550 °C/2 h	1.0 gL−1	30 min	500 W Xe lamp 75 min	100 mL of 5 mg/L (CIP)	90.33%	[73]
B1T1 (Biochar–TiO2 (1:1)) TiO2	coffee grounds 650 °C/2 h	1 g/L 0.1 g	60 min	125 W mercury vapor lamp UV irrad 120 min	DCF pH = 6.15 20 mg/L	90% 40%	[77]
TiO2 Ti-RSB	Rice straw 800 °C/1 h	0.5 gL−1	30 min	UV light 150 min	10 mgL−1 (CIP) pH = 5	83.82%	[98]
Fe/Ti biochar composite (Fe/Ti-MBC)	Wood chip	0.25 g/L	12 h	6 h sunlight, xenon lamp, ultraviolet lamp	40 mg/L CIP NOR pH = 6	88.4% 88.0%	[99]
TiO2/activated carbon	Lignin reed straw	0.25 g/L	16 h	Suntest solar simulator, Xe lamp 6 h	5 mg/L ACE pH = 6.9	92%	[91]
BC-TiO2 (300) acid pre-treated	Reed straw	1.25 g/L	30 min	50 W xenon lamp UV 3 h	10 mg/L SMX pH = 4	91.27%	[101]

NOR—norfloxacin; ENR—enrofloxacin; SMX—sulfamethoxazole; CIP—ciprofloxacin; ACE—acetaminophen; DCF—diclofenac.

provides an overview of selected recent research related to the photocatalytic degradation of antibiotics. From the results presented in Table 2, it can be concluded that biochar-supported photocatalysts exhibit extremely high efficiency for the photodegradation of various antibiotics, mostly in the range of 80–90%. As expected, the efficiency depends on the irradiation source used and increases with the irradiation time.

6.2. Dyes

He et al., 2021 [102] used a hydrothermal method to prepare photocatalytic composites consisting of biochar-supported ZnO nanoparticles. The optimal molar ratio of biochar/ZnO was 1:0.5 for degradation of methylene blue (MB) under UV–visible light. Based on the results of transient photocurrent response and the electrochemical impedance spectroscopy (EIS), they claim that at the interface between the biochar and ZnO nanoparticles, the chemical bond Zn-O-C was responsible for the lower recombination rate of the electron–hole pairs generated after UV-vis irradiation of photoactive ZnO species. The higher photocatalytic efficiency in the degradation of MB was exhibited by the prepared composite compared to the pure ZnO. The reaction mechanism took place through the formed reactive radicals such as holes, superoxide anion radicals, and hydroxyl radicals.

de Lima Brombilla et al. [103] have recently prepared a TiO₂/biochar composite by impregnation of biochar with TiO₂ and used it for the photocatalytic degradation of rhodamine-B (RhB) in water. Biochar was derived from Ilex paraguariensis. Their reported results showed that the prepared composite showed superior photocatalytic activity compared to pure TiO₂ with 96% decolorization and 75% TOC removal within 180 min of irradiation. The better photocatalytic activity of the composite was attributed to the increased surface area, the uniform distribution of TiO₂ on the surface of the support, and the reduction in the bandgap energy (3.11 eV).

Other reported works also found that a biochar nanoparticle significantly improves TiO₂ photoactivity in the degradation of MB. The biochar nanoparticles over a TiO₂ nanotube (BC-TiO₂) under UV light (365 nm) increased the rate of photocatalytic degradation of MB approximately three times with respect to bare TiO₂. The complete mineralization of MB has been conducted with 5-MBC-TiO₂ in 48 h, while nutshell-derived materials and bare TiO₂ were unable accomplish the task, respectively. They assumed that the photocatalytic process takes place on the surface of the prepared materials through a combination of adsorption reactions on BC nanoparticles and oxidation reactions that take place due to the presence of holes on the surface of TiO₂ nanotubes. They also believe that the degradation process is enhanced for samples showing isolated BC nanoparticles, and that the formation of a BC film leads to a decrease in photocatalytic activity due to a reduction in the surface area of TiO₂ available for the oxidation process [104].

Hosseini-Monfared et al. prepared a nitrogen-doped TiO₂/biochar nanocomposite (N-TiO₂/C) for the photocatalytic degradation of rhodamine-B (RhB) under visible light [92]. Barley straw was used to prepare the biochar. The prepared composite was able to degrade 99% of RhB. It was found that during the photodegradation of RhB by the N-doped TiO₂/C composite, the color changes from magenta to green, then to yellow, and finally becomes colorless. The N-doped TiO₂ sample with a BET surface area of 220 m² g⁻¹ and a mesoporous surface area of 80 m²g⁻¹ eliminated 94% of the RhB after 150 min of irradiation. The presence of 3% biochar improved the mesoporous surface area to 98 m² g⁻¹, and dye elimination was increased by more than 3% in 45 min shorter irradiation time. This result for N-TiO₂/C can be explained by its larger surface area, larger number of active sites for RhB adsorption, porosity, and large number of surface hydroxyl groups. On the other hand, calcination obviously led to a decrease in the BET surface area of 80 m²g⁻¹ and mesoporous surface area of 23 m²g⁻¹ of the same composite and a significant decrease in RhB elimination, which was only 22%, although the irradiation time was extended by 30 min.

Table 3. Biochar-supported photocatalysts for degradation of dyes.

Photoactive Specie/ Composite	Biomass/Pyrolysis Temp. °C/Time	Mass of Photocatalyst	Adsorption Eq. Time	Light Source/ Irradiation Time	Pollutant	Decolorization, %/Mineralization, %	Ref.
BTiO2	yerba mate/650	0.1 g/L		UV lamp (F15T8/B, GE 15 W, λ = 380–445 nm/ 120 min	(RhB) pH = 6.5	96%/75%	[103]
TiO2 nanotubes 5-NBC-TiO2	commercially available feedstocks: nutshells (NBC) and the microalgae Nannochloropsis sp. (MBC)/350/1 h	0.1 g/L		UV LED 40 mW/cm2 λ = 365 nm 3 h	(MB) 10 mg/L	5-MBC-TiO2 complete mineralization, 48 h 90%	[104]
5-MBC-TiO2		1 g/L		100 mW/cm2 λ = 365 nm 3 h	10 mL (MB) 10 mg/L	90%
0.2:1 biochar: Ti ratio, labeled as CT0.2/1	walnut shells	1 g/L	60 min	A mercury lamp, 500 W, λ = 360 nm	MO 20 mg/L	96.88% 83.23%	[74]
N-TiO2	barley straw	1 g/L	30 min	Vis LED lamp 150 min	10 ppm RhB	99%	[92]
N-TiO2/with 3% C	barley straw	1 g/L	30 min	105 min	RhB	97%	[92]
Calcinated N-TiO2/with 3% C	barley straw	1 g/L	30 min	180 min	RhB	22%	[92]
TiO2/BC	coconut shell	6 g/L	not reported	60 min	RBB KN-R 30 mg/L	pH = 1 (99.71%) pH = 11 (96.99%)	[105]
hybrid biochar-TiO2 (BCT)	macroalgae	2 g/L	60 min	3 h Vis light	MB 50 mg/L	99.40%(BCT-4) 41.30	[59]
Ag/TiO2/ biochar (CTAg1)	walnut shell WB700	0.25 g/L	60 min	1 h 500 W mercury-vapor lamp	MO 20 mg/L	97.48% DE 85.38% ME	[106]
PCT-400–550	grapefruit Peel	0.5 g/L	30 min	30 min 300 W mercury lamp	RhB, 10 mg/L 100 mL pH = 4.9 MO, MB	99.07%	[107]
3% TiO2@PNS-BC	pistachio biochar (PNS-BC)	0.5 g	Not reported	30 min UV lamp	MO 150 mL 100 ppm pH = 6	99.47%	[108]
2% TiO2@PNS-BC						88.44%
						81.38%
1% TiO2@PNS-BC

RhB—Rhodamine-B; MB—Methylene Blue; MO—Methyl Orange; RBB KN-R—Reactive Brilliant Blue KN-R.

provides an overview of selected recent research related to the photocatalytic degradation of dyes.

Zhang et al., 2018 [105] synthesized an economical and highly efficient new TiO₂ photocatalysts supported on coconut shell biochar through the sol–gel method. To prepare the biochar, coconut shells were first pyrolyzed at 450 °C for two hours under a nitrogen atmosphere. Then, the obtained BCs were washed with distilled water and impregnated with 0.1 M HCl. After 12 h, they were rinsed with distilled water and dried at 105 °C. To prepare the TiO₂/BC composite by the sol–gel method, a mixture of ethanol, distilled water, and acetic acid in a ratio of 6:1:3 was mixed with 6 mL of polyethylene glycol, and this mixture was then added dropwise to the mixture of ethanol and butyl titanate in a ratio of 2:1 and mixed for 24 h. The resulting homogeneous sol–gel was mixed with the BC precursor for 4 h, then left for 24 h, filtered, then dried for 12 h at 80 °C, and calcined at 450 °C for 3 h. They studied the photocatalytic degradation of a typical anthraquinone dye (Reactive Brilliant Blue KN-R) under a UV high-pressure xenon lamp (300 W). Almost complete degradation was achieved in 60 min at pH = 1 (99.71%) and pH = 11 (96.99%), respectively.

7. Discussion and Future Prospective

Water and air pollution, soil degradation, and biodiversity loss are serious threats to ecosystems and require urgent solutions. There are various approaches to mitigate these risks and promote sustainable development. Based on existing knowledge, photocatalysis is considered a priority approach as it utilizes readily available resources such as waste biomass and solar energy. The main advantage of biochar-based photocatalytic composites over other photocatalytic materials is that they enable the simultaneous utilization of waste biomass and the efficient removal of priority pollutants from the environment. Despite great progress in this area of research, there is still much room for further improvement. This applies in particular to the following aspects: (i) there is little information in the literature on hazards and risks regarding the potential release of toxic substances during the preparation of biochar-based composites, depending on the biomass source; (ii) thermochemical conversions of biomass are still energy-intensive processes as they are carried out at high temperatures, (iii) there is a need for further research on the possibility of further activation and optimization of the properties of such materials, improving their long-term stability and reducing possible metal leaching as well as the possibility of their recovery, (iv) the mechanism of decomposition of various pollutants is not yet fully understood and needs further research, (v) most research has been carried out in closed systems (batch reactors), but there is insufficient information on other types of chemical reactors that enable the use of a broader spectrum of solar irradiation and operation in dynamic flow systems, etc.

This review comprehensively addresses the problem of the presence of organic pollutants in water (especially antibiotics and dyes), but also offers a simple, environmentally friendly, and sustainable solution based on the synthesis of new photocatalytic composite materials based on TiO₂-biochar. In the introductory part of this paper, we have seen the enormous amount of municipal wastewater generated as a result of population growth, the improvement of standards, and economic growth. We also saw that it is estimated that a new generation of wastewater will emerge by 2050 with an increase of 51%. In this work, dyes and antibiotics were selected as polluting components because they represent a threat to the living world and the environment, even if they are present in very low concentrations in the water.

Water purification with biochar-based photocatalytic composites is based on two processes: adsorption and photocatalysis. The results of the various studies indicate that this method of treating polluted water is an environmentally friendly and sustainable technology. The role and benefits of biochar as a support for photoactive species are explained in detail. Agricultural and food wastes, animal and poultry manure wastes, industrial wastes, and algae and crustacean wastes are studied as raw materials for the production of biochar. The possibilities and methods of biochar preparation as well as its structural and physico-chemical properties, and the importance of the presence of functional groups on the surface of the biochar, are also presented. The advantages of a composite of biochar and a photoactive species, i.e., a higher photocatalytic activity than in the separate presence of these two components, are particularly emphasized. The reaction mechanism of photocatalytic degradation of organic pollutants is presented. We have seen that biochar-based photocatalysts are robust composite materials, but further research with a view towards larger production and their use in real-life contexts is very important, as it is certainly a sustainable and environmentally friendly solution to the existing problem.

8. Conclusions

In this paper, it has been shown that different types of waste, but mainly plant residues and animal and industrial waste, can be successfully converted into biochar using different technologies, and that such a new product can be used as a support for photoactive species for the production of robust and highly efficient composite photocatalysts for the removal of organic pollutants. The use of agricultural biomass residues to prepare biochar instead of expensive disposal as waste could improve the sustainability and economics of photocatalytic support production. It is recognized that the use of semiconductors on different substrate types can improve porosity and surface area, facilitate separation for later reuse, and reduce bandgap energy, all of which have a positive impact on the photocatalytic process.