Biowaste Valorization Using Hydrothermal Carbonization for Potential Wastewater Treatment Applications

Abstract

In today’s world, due to population increase, there are many alarming and potential catastrophic problems like climate change, environmental pollution and an enormous mass of wastes constantly produced by humankind to find innovative solutions for the management, recycling, and valorization of biowaste from agricultural production, food processing, and organic household residues. The search for sustainable and efficient wastewater treatment technologies has gained scientific interest recently; particular focus is on using biowaste to produce hydrochars (HCs) via the hydrothermal carbonization (HTC) process used as adsorbent materials for dye, heavy metal, and emerging pollutant removal. HTC materials derived from renewable resources are an environmentally friendly and adequate way to adsorb pollutants such as organic and inorganic molecules from wastewaters. This review focuses on the advantages of the HTC process which lead to improved properties of the materials obtained, making them highly efficient in wastewater treatment. The information presented in this paper was derived from the most recent publications in the field. Future perspectives of HC materials should consider the possibilities of scale-up, pretreatment of biowastes, and the optimal parameters of the HTC process to produce HCs applied for pollutant removal from wastewaters.

1. Introduction

Due to some alarming and potential catastrophic problems, such as climate change and the enormous mass of waste produced constantly, humankind has been forced to find innovative solutions for the management, recycling, and valorization of biomass from by-products of agricultural production, food processing, and organic household residues.

Knowledge based on the conversion of vegetal side streams into bio-based products and services is part of a global view called the bioeconomy concept [1], whereas aims to leverage the innovations in (life) sciences and bio-industries by moving from fossil to renewable resources [2]. In 2015, by reconsidering the issues of bioeconomy in direct relation to the circular economy, these two concepts were merged into the term circular bioeconomy, with a significant impact on biowaste management [3,4].

One of the main focuses of the circular bioeconomy is the valorization of biomass in integrated production, as a sustainable, efficient resource, using waste and optimizing its value over time, as shown in Figure 1. Such optimization emphasizes the environmental, economic, and/or social aspects, considering all three pillars of sustainability.

Biomass represents the raw material of bioeconomy; visionaries consider that the future will bring many biomass-derived products, such as biofuels, “green chemicals”, and biomaterials. The proportions of cellulose, hemicellulose, and lignin in biomass range from 40% to 50%, 25% to 30%, and 15% to 20%, respectively, but depending on the type of biomass, the concentrations of these components vary. Through the thermal treatment of biomass, biochar is produced; its properties depend on the reaction conditions (temperature, reaction time, the medium of the reaction), in addition to the composition of the biomass used.

Some of the most common technologies used for the management and treatment of biomass waste, together with the potential to produce bioenergy, biofuels, polymers and value-added chemicals in the context of circular bioeconomy, are: land fields, composting, thermochemical conversion processes (such as gasification, liquefaction, and pyrolysis) and biochemical conversion methods (such as fermentation, anaerobic digestion, and microbial fuel cells) [6]. Some disadvantages of these technologies require long reaction times, constant and costly maintenance of the equipment used, high energy consumption, huge carbon footprint, and expensive transport and storage costs [7]. The hydrothermal carbonization (HTC) process can overcome some of these problems, having benefits of lowering and/or eliminating the volume of organic waste which is often landfilled due to lengthy transportation routes, lower energy consumption during the process, lower cost because it uses water as a solvent, and other factors thus making it a viable technology which can be integrated in the circular bioeconomy flux [8].

Due to its rigorous treatment at high temperatures and, in certain cases, high pressure, thermochemical conversion is regarded as one of those methods that can treat any sort of feedstock [9]. The end products of this method can be broken down into three categories: solid, liquid, and gaseous. The ratios of the three fractions can be changed by adjusting the processing parameters (temperature, pressure, etc.), the reaction medium (O₂, N₂, and CO₂ gases, water/liquid solvent, etc.), and the catalysts. In general, the temperature of the process ranges from 200 °C to 1600 °C and the pressure can range from atmospheric to as high as 400 bar [10], thus demonstrating the wide range of the thermochemical conversion process.

The treatment and management of solid waste, including biomass, are a global environmental and social challenge. However, these wastes can become raw materials for energy and value-added products. In addition to the conventional approaches of directly applying a thermal treatment method to biomass for energy production, the emerging approaches applied today include: (i) thermal treatment system modelling, (ii) the conversion of non-wood lignocellulosic biomass into value-added products, (iii) solid waste management, (iv) computational approaches for application in biotechnologies, and (v) carbon-based value-added materials [11].

Recently, there has been an increase in scientific interest in the search for a sustainable and affordable wastewater treatment technology. Due to their abundance and simplicity of availability, particular focus is on the use of sustainable and inexpensive adsorbents [12]. However, one common issue with crude adsorbents is the leakage of organic compounds, which can lead to secondary pollution and water oxygen depletion. Techniques to overcome these issues have been suggested, such as carbonization or charring [13]. Pyrolysis has long been the most popular conventional thermal process for carbonizing biowastes and natural resources [14]. Biochar is produced by this process at high temperatures (>400 °C) in an oxygen-free environment, and activated carbon is created by changing the physical and chemical properties of the organic matter at temperatures above 800 °C [15].

For many years, activated carbon has demonstrated its high efficiency in removing pollutants from wastewater treatment facilities and the food sector. Additionally, it is used to discolor syrups, vinegar, wine, refine sugar, and remove methylene blue (MB) and other dyes [16]. However, the high cost of manufacture restricts the use of activated carbon, attributable in part to the pyrolysis method and its application [17]. This limitation has driven the pursuit of a simpler and more lucrative technique for the carbonization of biowastes called hydrothermal carbonization (HTC) [18].

The HTC process usually occurs at temperatures ranging from 180 °C to 260 °C, time of residence between 0.5 h to 24 h, in the absence of air, leading to high autogenous pressures of 25–60 bar, using water as a reaction medium. Exothermic reactions require little heat energy, are very flexible, and can utilize a wide range of feedstock regardless of moisture content or heterogeneity. The HTC process is very complex, consisting mostly of reactions such as decarbonylation, decarboxylation, dehydration, hydrolysis polymerization, and re-condensation [19]. These reactions lead to a solid phase containing hydrochar (HC) with properties similar to fossil coal, a liquid phase where valuable chemicals such as furfural (FF) and hydroxymethyl furfural (HMF) can be found, and a gaseous phase consisting mainly of CO, with CO₂ occurring mostly from the decarboxylation reactions. A graphical representation of the HTC process is shown in Figure 2. Additionally, the use of an appropriate catalyst can greatly influence the yield in HC, as well as other valuable chemicals present in the liquid and gaseous phases. The HCs are attractive materials which can be used for many purposes such as fuel, supercapacitor, soil amendment processes, adsorbents, catalysts, and nanostructured materials [20,21,22].

Dairy manure and wastewater sludge digestate [23], wet biodegradable residues [24], sewage sludge [25], activated sludge [26,27], hazelnut shells [28], and cigarette butts [29] are examples of biowastes investigated by the HTC process. Lignocellulosic biomass (LCB), frequently called simple lignocellulose, represents the plant dry matter, which is the Earth’s most plentiful raw mineral. Structurally, LCB is a complex mixture of carbohydrate polymers (cellulose, hemicellulose) interconnected with an aromatic polymer (lignin). In addition to these biopolymers, LCB contains many other types of bio-compounds such as polyphenols, alcohols, phenols, and phyto-acids.

The use of HC from biowaste recovered through the HTC process to remove environmental pollutants, such as persistent organic pollutants and heavy metals from industrial wastewater, has recently developed into a technology with enormous research potential [30].

Figure 2. Illustration of the HTC process for different wastes and the generation of HC adsorbents (adapted from [31]).

Figure 2. Illustration of the HTC process for different wastes and the generation of HC adsorbents (adapted from [31]).

It has been found that because of the speed and the modularity of HTC processes, scaling up can be achieved by increasing the number of reactors installed close to the waste source, thus eliminating the odor problems [8]. In this case, the CO₂, CH₄, and C emissions are reduced due to shorter distance transport of waste. Following IPCC (Intergovernmental Panel on Climate Change) recommendations for calculating CO₂ emissions, it was estimated that the total CO₂-eq (CO₂-equivalent) emissions avoided were between 6.5 and 8.4 tons of CO₂-eq/ton of hydrothermal coal [8].

The aim of this review paper was to critically review the available literature on the treatment of biowastes via the HTC process to obtain adsorbent materials with adequate properties, which recommend the use of HCs in wastewater treatment. Our paper is structured as follows: in the first part, the main characteristics of the HTC process reviewed in the literature from 2000 until now are evidenced; in the second part, the study focuses examples reported in the literature of applied HCs in water treatment; finally, our recommendations for improving the state of current knowledge on the potential use of the HTC process for biowaste management in the bio-economy concept are provided.

2. Characteristics of the Hydrothermal Carbonization (HTC) Process

HC presents different properties in comparison to pyrochar (PC), the biochar obtained from pyrolysis of biomass, due to the reaction mechanism involved: in the course of HTC, the carbohydrates are hydrolyzed and the intermediates form the HC by polymerization, but during the pyrolysis process, the structural components of biomass decompose, yielding volatiles and the PC. Some advantages that the HC and PC present over fossil charcoal are their stable products, chemical purity, because they do not contain mercury or sulfur, a low content of nitrogen and ash, and a lower degree of aromatization. Therefore, HC and PC have a higher reactivity and porosity than fossil charcoal [32]. Among all biochar, due to the similarities with coal [33,34,35], HC can be applied for energy production, soil amelioration, and as an adsorbent in water treatment processes [36]. The HTC process reduces the hydrogen and oxygen contents in the biomass, due to the decarboxylation and dehydration reactions occurring at selected temperatures in the water medium.

Water has a key role in the HTC process due to its adjustable physical and chemical properties, such as density, hydrogen bonding, ionization constant (K_w), and dielectric constant (k). Depending on the temperature and pressure, these properties are essential in the activity of the biomass to break the C–O or C–C bounds. Water acts as a catalyst and solvent in the HTC process; therefore, very few studies have been performed with additional catalysts. In most cases, weak organic acids, such as citric acid and acetic acid, are used to enhance the HTC process and to obtain HC with improved properties.

In their studies, Amin Ghaziaskar et al. [37] enhanced the HTC process by recycling the process liquid, which exhibited catalytic activity and enhanced the energy yield and elemental composition of the HC; additionally, a higher heating value was observed for the HC from the catalyzed process when compared with the HC obtained from the uncatalyzed reaction. The catalytic activity of the process liquid was strongly related to the organic acids that form during the process, which led to a lowering of the pH, enhancing the dehydration and condensation reactions in the acid-catalyzed process and the HC production.

The biomass processed by the HTC process involves several complex reaction pathways. There is a wide range of information available in the literature on possible chemical reactions that occur in the HTC process. Unfortunately, few studies have conducted in-depth analyses of the chemical pathways; some examples refer to the hydrolysis of cellulose. Decarboxylation and dehydration reactions combine to produce this reaction [38,39,40], which is recognized to be exothermic [40,41,42]. However, as of now, the complex reaction mechanisms are still not fully known. Reaction pathways documented in the literature originate from analogous networks of other reaction pathways. However, they are recognized for offering crucial knowledge regarding reaction mechanisms and are divided as follows: (i) hydrolysis (decomposition); (ii) decarboxylation and dehydration; (iii) condensation polymerization (recombination); and (iv) aromatization. Although the mechanisms of these reaction pathways rely on the structure and type of biomass waste utilized [43], achieving an understanding of the general reaction mechanisms of the HTC process in biowaste is important for further research.

Parameters influencing the HTC process include the biomass solids content, temperature reaction, reaction medium and catalyst, reaction time, pressure, and pH used. They are discussed as follows.

2.1. Effect of Solid Raw Material Content

Due to their reactivity, furfural and 5-(hydroxymethyl) furfural (5-HMF) obtained by the dehydration of pentoses and hexoses present in the different biowastes are crucial in the synthesis of HCs. So long as these chemicals are present in the reaction medium, it is expected to obtain HC materials with comparable morphology and structure, despite the precursor used. In their study, Titirici et al. [44] showed that all carbon spheres derived from hexose and 5-HMF have similar morphology. Additionally, carbon spheres derived from furfural and xylose via the HTC process are indistinguishable as shape. Falco et al. [45], who studied the morphology of HCs from cellulose, glucose, and rye straw, provide more support. Carbon spheres made of glucose and cellulose appear to have a similar shape; however, it is believed that long cellulose fibers are destroyed during HTC processes, causing the development of shorter chains that take on a spherical shape to diminish contact with water.

Some studies have reported that because the raw material used in the HTC process contains a mixture of complex molecules and compounds (life biomass wastes), the morphology of the final product varies. For example, Niinipuu et al. [46] used horse manure, fiber sludge, bio-sludge, and sewage sludge as raw material in the HTC process at 180 °C, 220 °C, and 260 °C; from the SEM images, it was revealed that all the HC produce had different morphologies depending on the raw material used. For horse manure HC, the structure bears a resemblance to that of lignocellulosic biomass, and the sewage sludge HC exhibited amorphous particles intermixed with fibers, whereas only large amorphous particles were observed for bio-sludge-derived HC. In the case of fiber sludge, only fiber structures with a similar appearance were observed.

The morphology of HCs produced using corn stalks as solid raw material in the HTC process at 200 °C and different reaction times was investigated by Lei et al. [47]. Corn stalks contain a large amount of cellulose and hemicellulose (47.98% and 21.78%); thus, the final HCs produce spherical particles over an 8 h reaction time.

Usually, the pretreatment of biowastes is required. For example, dried avocado seeds were mixed with NaOH 0.1 M, under reflux, over a 2 h residence time and at a temperature of 70 °C to eliminate dyes and tannins before separation and washing [48]. Citric acid was added together with Ziziphus mauritiana L. fruit pulp and distilled water for carbonization under acidic conditions [49].

2.2. Effect of the Temperature

Numerous studies and reports have been conducted regarding the impact of reaction temperature for biowaste processed by HTC. It has been found that temperature is the key element influencing the product characteristics of HTC [17,50,51]. The reaction temperature has a significant impact on HC characteristics, presenting an almost-linear relationship with carbon content; thus, with a temperature increase, the carbon yield decreases [52,53]. The temperature increased the amount of glucose in the water-soluble component which, at higher temperatures, further broke down into 5-HMF [54]. The hydrolysis rate of biomass fragments also depends on the temperature [55,56]. Hemicellulose degrades from 220 to 315 °C, followed by cellulose which decomposes in a temperature range from 315 to 400 °C, and lignin at over 220 °C [57]. The temperature also affects the rate of polymerization [58].

The effects of different temperatures (200 °C, 225 °C, and 250 °C) and processing times (4 h, 6 h, and 8 h) on HC processed from oil palm shell (OPS) were studied by Budiman et al. [59]. Based on the best properties obtained by the determination of the amount of iodine and surface area of HC, the authors concluded that a temperature of 225 °C and a residence time of 8 h are the optimal parameter values for conducting the HTC process.

2.3. Effect of the Pressure

During the HTC process, pressure is self-generated and rises automatically. Higher reaction pressures result in a decrease in hydration and decarboxylation processes, which are often the main mechanisms in this process [17]. Nevertheless, it has been found that this effect has a negligible impact on HTC and natural charring [60].

2.4. Effect of Water as a Catalyst

The literature on biomass reactions in water has been widely reviewed and reported. Water can be a solvent and catalyst during the biowaste HTC process, and it has been observed to accelerate the carbonization process [61]. Water is more ionized at higher temperatures, which allows for hydrolysis, ionic condensation, and splitting; therefore, it acts as a reaction medium and a catalyst for the formation of organic molecules [62]. Additionally, water’s dielectric constant strongly decreases in HTC conditions, whereas its self-diffusion coefficient increases. As a result, under these circumstances, water behaves as a polar solvent with some organic characteristics. The properties and distribution of the product are influenced by how much water is present in the biomass feedstock [63].

It has been observed that the addition of HCl to the HTC water leads to the increase in adsorption efficiency of HC, lowering the temperature and reaction time [64]. In comparison with HC produced without acid, HC obtained in the HCl-assisted HTC process presented more oxygenated functional groups, higher aromaticity and hydrophobicity, larger pores, and superior thermal stability. Furthermore, in the case of sugar bagasse transformed into HC for crystal violet dye and tetracycline removal, the maximum adsorption capacities of 207.16 mg/g and 68.25 mg/g, respectively, were recorded by the HCl-assisted HTC process.

2.5. Effect of the Reaction Time

The qualities of the product were discovered to be impacted by the reaction time [47]. Low reaction times and temperatures (<200 °C) result in a structure with a higher proportion of furan groups, whereas lengthy residence durations (above 24 h) and high temperatures (>200 °C) result in an arene-rich structure, which is either the product of condensed polynuclear aromatic hydrocarbons structures or three-membered furan units [45]. Longer reaction times were observed to significantly increase the yield of HC [39], which contrasts with the lower yield obtained at longer reaction times [17], [52]. Heilmann et al. [53] advised to use shorter reaction times after observing that the reaction lengths do not really affect HTC.

2.6. Effect of the pH

It was observed that the pH decreases during the HTC process because of the formation of organic acids, such as acetic, formic, lactic, and levulinic acids [22,40]. The decomposition and hydrolysis of biomacromolecules are catalyzed by organic acids, generating several intermediates to enhance hydrothermal carbon production [65]. This behavior may also be attributed to the high ionic strength brought on by the higher proton ion concentration in the process water, which may have the ability to speed up reaction rates and modify reaction paths to produce the specific HCs [66]. Therefore, studies have reported the addition of acids to catalyze the HTC process [67]. Particular focus is placed on the addition of external acid, because the type of acid can influence the dissolution of macromolecules to varying degrees, subsequently affecting the conversion process [68]. For instance, cellulose’s hydrogen bonding can be broken using hydrochloric acid rather than sulfuric acid to speed up its dissolution [69].

In some studies, it was reported that regardless of the starting pH of the feed water, both solid (HC) and liquid phases produced during the HTC process presented an acidic pH [70,71]. It was observed that the composition of HC and the chemical components in the liquid phase vary with pH. In their study, Reza et al. [70] prepared HC from wheat straws 200 and 260 °C in both acidic and basic conditions using acetic acid and potassium hydroxide (KOH). They reported that at a pH of 12, the HC produced had a smaller surface area, pore volume, and size than that produced at pH 2. Additionally, at a basic pH and a temperature of 260 °C, the content of phenolic compounds and organic compounds increased, whereas the sugar content decreased.

In another study, sewage sludge was treated via the HTC process at 270 °C for 2 h, and a pressure of 7–9 MPa [72]. The effects of feed water pH (2–12) on the organic component, thermal behavior, and characteristics of the HC produced were investigated. It was found that at a basic pH, the HC presented a cambium lamellar structure, whereas in acidic conditions, the spherical structure was observed. It was also reported that the phenols content, ketones, and nitrogen-containing compounds were higher with the increase in pH value. In addition to having an effect on the HTC process, raising the pH value (>2) could marginally but positively stabilize the amount of heavy metals in the hydrothermal carbon derived from sewage wastes [73]. HC is utilized as a soil supplement; therefore, pH variation is not a good way to reduce the toxicity of heavy metals found in HC.

The physical properties of the final product are significantly affected by the pH as well. An enhanced surface area and higher porosity can be attained as a result of the higher development of microspheres on the HC surface, for instance, because lower pH conditions may expedite the hydrolysis of carbohydrates from cellulose [74]. The surface of HC derived from sludge becomes negatively charged at basic pH values, promoting its adsorption capacity and removal efficiency for the pollutants [75].

2.7. Effect of the Raw Material–Water Ratio

Another factor that affects the final product from the HTC process is the raw material–water ratio. It has been reported that, in general, the amount of water is between 3 and 10 times higher than the amount of raw material [76], and an increase in the water content led to the decreased yield in HC [77]. This phenomenon can be attributed to the greater impacts the hydrolysis reactions caused by the increase in water quantity have on the raw material [78,79]. Oktaviananda et al. [80] investigated the raw material–water ratio effect on the final product when using sawdust as the raw material. It was treated with the HTC process at 200 °C and a raw material–water ratio from 5% to 20%. The authors observed that with an increase in the raw material–water ratio, the yield in HC also increased from 63.11% to 69.57%. This means that the raw material degradation decreases at a higher concentration of biomass due to the lower dissolution power of the reduced content of water.

3. Utilization of HCs for the Decontamination of Wastewaters

In order to prevent and control the pollution of surface water and groundwater resources, the governments of many developed and developing countries have established various firm rules and regulations to maintain the quality of available water resources. In practice, preventing and controlling water pollution is different from controlling water quality. Water quality standards may differ depending on the intended use of the water; pollutant limits may be prescribed depending on the primary use, such as drinking, cooking, washing, bathing, and agricultural work [81]. In some cases, these limits and standards are not properly enforced unless they are closely related to human consumption. Governments regulate the water quality by implementing general rules, which will help to avoid water quality decline and maintain natural states.

Due to their unique qualities, some wastes, such as biosolid (sewage sludge), biowaste (for example, compost from municipal waste), or fly ash from burning coal, are suitable for use on land as a nutrient source and organic matter, or as a soil amendment. Human activities can lead to the conversion of soil into a kind of reservoir for the accumulation of all possible chemicals, such as xenobiotics (halogenated organic substances (AOX), di(2-ethylhexyl)phthalate (DEHP), polychlorinated biphenyls (PCB), organo-tins (MBT, DBT, and TBT), organic dyes (methylene blue (MB) [82,83,84,85], malachite green (MG) [86,87,88], crystal violet (CV) [75,89], Congo red (CR) [87,90], and rhodamine B (RB) [91,92]), heavy metals [93,94,95,96,97] and metalloids and polycyclic aromatic hydrocarbons (PAHs), and various pharmaceuticals [33], which can leakage from soil to water sources. Due to their high toxicity and lack of biodegradability, these types of pollutants have drawn considerable attention when it comes to polluting water.

HC materials made from sustainable resources have a variety of known uses, including an effective and eco-friendly method of removing contaminants from water. They have been shown in numerous studies to be capable of adsorbing both organic [98,99,100] and inorganic molecules [101,102,103].

3.1. HCs for Organic Contaminants Retention from Wastewaters

In this part of the review, the HCs have been divided into two categories: those used for the elimination of dyes from wastewater, and others used for emerging pollutants.

3.1.1. Dyes

Dyes interfere with bacterial growth and prevent aquatic plants from producing oxygen through photosynthesis; therefore, they are an environmental concern [104]. Even at low concentrations, these phenomena can be seen. The primary source of the dyes is industrial effluent. The elimination of organics and the removal of metal ions are two separate processes. Carbon-based adsorbents have been explained by three main mechanisms: (i) hydrogen bonds, (ii) π–π interactions, and (iii) the creation of donor–acceptor complexes [105,106]. Another possible adsorption mechanism may occur between electrostatic and dispersive interactions [107]. The amphoteric characteristics of carbonaceous materials have an immediate impact on the adsorption mechanism [108]. These characteristics largely depend on the content of heteroatoms, which determines the electron density, hydrophobicity, and surface charge [109].

Reactive dyes are thought to be released into downstream effluents in amounts of 200,000 tons per year [92], leading to some cases in which dye concentrations in aqueous effluents can reach values up to 800 mg/L. Treating these dye-containing effluents poses considerable problems in the wastewater industry. From an ecological perspective, it is crucial to remove dyes from water sources [86]. The HTC process can be used to transform various biowastes into HC for elimination of dyes from wastewater.

Valuable biowastes, such as coconut shells [110], coffee husks [111], hazelnut shells [112], chickpea stems [113], shrimp shells [114], cigarette butts [29], and forestry [115,116,117], have been exploited to produce HCs for dye removal from aqueous waters. Several recent studies that have used the HTC process on biowastes to produce adsorbents for organic pollutants from wastewaters, process conditions, and adsorption performance in terms of adsorption capacity (Q_e) and removal efficiency are presented in

Table 1. HTC conditions and adsorption performance of HC for dyes removal for wastewater treatment.

Biowaste	HTC Conditions			Adsorption, Pollutant Retention Efficiency		Ref.
	T (°C)	Residence time (h)	Activation	Dye	Qe (mg/g)/Removal (%)
Pine needles (PNs)	225	5	-	MG	52.9/92	[116]
			H2O2		97.1/96
Pine wood	300	4	NaCl	MB	86.7/n.a.	[93]
Bamboo	180	24	-	MB	91.74/n.a.	[118]
Pretreated cotton stalk	220	6	KOH	MB	198.0 ± 9.8/n.a.	[119]
Coffee husk	180	6	KOH	MB	357.38/n.a.	[111]
Sugarcane bagasse	240	10	-	MB	116.65/n.a.	[82]
			NaOH		334.74/n.a.
Hazelnut shell	250	7.5	KOH	MB	524/n.a.	[112]
Chickpea stem	200	5	KOH	MB	96.15/77.86	[113]
Coconut shell	200	2	NaOH	MB	200.01/98	[110]
Shrimp shell	180	12	Acetic acid	Methyl orange (MO)	755.08/n.a.	[114]
Phycocyanin-extracted algal bloom residues (PE-ABR)	200	10	-	MG	89.05/92.4	[111]
Avocado seeds	230	3	-	Indigo carmine	49	[48]
Sewage sludge	180	3	-	CV	n.a./99	[75]
Cigarette butts (CBs)	190	48 and 72	NaOH	MB	561.73 and 548.72/n.a.	[29]
PVC + bamboo	200	24	NaOH	MB	234.46/n.a.	[120]

.

As observed from Table 1, most studies refer to the removal of MB from wastewater with the help of HCs derived from agro-residual wastes [82,111,112,113], woody biomass [94,119], and composites based on PVC and bamboo [120].

In a study by Roldan et al. [121], N- and S-doped mesoporous carbon prepared through a single-vessel HTC process were used for water treatment to adsorb MB and RB. They found that, generally, MB had a better adsorption capacity than RB, which may be mostly attributable to size-related factors, because the second contaminant has larger dimensions. Both molecules are cationic; therefore, there is little difference in how they interact with the HCs. The materials perform better before carbonization, explained by the lower degree of graphitization and the higher content of oxygen functional groups, which offers a more hydrophilic surface, improving the electrostatic interactions with MB and RB. By generating various pore sizes in the HCs, the dopant in this instance significantly enhances the adsorption capabilities. For porous S-doped carbon activated with ZnCl₂, the adsorption capacities were around 123 mg/g for MB and 106 mg/g for RB.

Alatalo et al. [122] studied the removal of MB from aqueous media using meso/microporous soft carbonaceous materials prepared by the HTC tempering method in basic media. The HCs were prepared from fructose and activated with a mixture of LiCl/ZnCl₂ in a single-vessel HTC at 180 °C. The first HC was prepared with pure fructose (FruLi) with polar oxygenated surface functionalities, and the second HC (FruLi + TCA) was made of fructose and 2-thiophenecarboxyaldehyde (TCA) with thiophene sulfur, the mixture being doped in the final HC structure. These HCs were subjected to MB adsorption tests in the pH range of 3–8, but only a minimal effect was seen. Additionally, Alatalo et al. examined the impact of temperatures between 20 and 60 °C. These studies revealed that the adsorption effectiveness somewhat increased between 20 and 40 °C, possibly as a result of a decrease in solution viscosity, which increased the diffusion rate of adsorbent molecules into the internal pores [98]. The maximum adsorption capacities for FruLi and FruLi + TCA were 96 mg/g and 64 mg/g, respectively, at equilibrium conditions (temperature 20 °C, pH 6, contact time 24 h).

A very high adsorption capacity for MB (735 mg/g) was reported by Correa et al. [123], using activated carbon made from various coals. A carbon–silicate composite produced by the HTC process was utilized by Xiong et al. [124] to remove MB, achieving a maximum adsorption capacity of 418 mg/g, which was significantly higher than what was found for HCs that were not activated or modified.

HCB48 and HCB72 hydrochars synthesized from 5 g CBs in 37 mL of deionized water at 48 and 72 h, respectively, without inert, were used as adsorbents for MB, as chemically activated (HCB48-ATV, HCB72-ATV) and non-activated forms [29]. The yield of HBCs was 26.81% (wt./wt.) for HCB48 and 23.95% (wt./wt.) for HCB72. The maximum MB removed occurred at pH 11.0 for activated HCB, at 20 min equilibration time, if the specific area of HCB activated was low (2.30 and 3.74 m²/g tested by BET) (Figure 3). The adsorption mechanisms were explained by the electrostatic interactions produced at high pH, due to the negative surface charge of adsorbents, which attracts the positively charged MB. Among Langmuir, Freundlich and Dubinin–Radushkevich models investigated, it was observed that the Langmuir isotherm better described the adsorption process.

In another paper, the HC derived from the HTC of chickpea stems, coded HTC-CSAC, was synthesized in the following conditions: 50–150% impregnation ratios of HC:KOH, 12–48 h impregnation time, activation temperatures between 400 and 600 °C, and 30–60 min activation times for MB removal [113]. The HTC-CSAC characterized by a high iodine adsorption number (IAN) of 887 mg/g, BET surface area of 455 m²/g, and an average pore diameter of 105 Å prepared at an impregnation ratio of 70%, at 600 °C, 45 min activation time, and 24 h impregnation time showed the high reuse efficiency after five consecutive adsorption–desorption cycles of 77.86%.

Zhang et al. [119] investigated the influence of pretreated cotton stalks with H₂SO₄, NaOH and H₂SO₄ + NaOH on the biomass HTC process, followed by the activation with KOH solution and the adsorption of MB. The purpose of pretreating the raw material with acid/base was to selectively remove the lignin and hemicellulose from cotton stalk, obtaining a high content of cellulose in the pretreated cotton stalk and enhancing the capacity of MB removal. It was demonstrated that the obtained HC had a low surface area and low porosity, and the treatment with H₂SO₄ + NaOH led to the high efficiency of MB removal. After four adsorption–desorption cycles, the MB adsorption decreased by 11% as compared with initial adsorption, suggesting the importance of abundant agricultural residual cotton stalk utilization for dyes removal.

Waste shrimp-shell-based HC (WSH), as an affordable adsorbent for MB removal, was successfully obtained by the deproteinization and deacetylation of waste shrimp followed by HTC and acid washing processes [114]. The adsorption experiment showed a significant adsorption capacity of MB up to 755.08 mg/g at 37 °C under an optimal pH of 4.0, explained by the electrostatic interaction.

An adsorbent for the removal of MB for wastewater treatment was developed by Wang et al. [118] using bamboo to produce HC via the HTC process. The authors optimized the process and found that 1108 μmol/g acid oxygen functional groups (AOFGs) and a carbon yield of 52.45% resulted in a better adsorption capacity and economic feasibility.

New adsorbents based on HCs synthesized from forestry waste and agro-residues such as bamboo (BHC), cedarwood (CHC), coconut shell (CSHC), mason pine (MPHC), maize straw (MSHC), pecan shell (PSHC), rice straw (RSHC), and wheat straw (WSHC) were studied by Zhang et al. [115] for MB removal from aqueous solutions. The following adsorption capacities for the above HCs were found: 155.14, 109.24, 93.15, 91.71, 88.11, 86.36, 70.01, and 64.43 mg/g, respectively, suggesting the high potential of biomass to be used as a low-cost adsorbent for wastewater treatment.

Based on these results, we conclude that all biowastes treated via the HTC process show promising adsorption capacities and removal efficiencies; thus, we recommend extending the research for obtaining HCs from low-cost and renewable resources for dye removal at industrial scales, as well as the activation of HCs.

3.1.2. Emerging Pollutants

Until recently, persistent organic pollutants, heavy metals, nutrients, and “conventional” active components in pesticides have received the most attention when discussing how chemicals affect the environment [125]. In recent years, concerns about the hazardous environmental consequences of so-called “emerging pollutants” (EPs) have grown. EPs can be found in a wide range of product categories, such as pharmaceuticals, veterinary medicines, nanomaterials, personal care products (PCPs), coatings and paints. Some EPs can develop in the environment by the biochemical processing of natural products in animals, plants, and microorganisms. In some areas, there are already environmental risk assessment procedures for these compounds.

Due to their presence in numerous water sources, EPs have drawn considerable attention. They come from an array of anthropogenic and natural sources, such as agrochemical treatments, forest fires, volcano eruptions, coal mining and processing, petrochemical manufacturing, leather and textile dyeing, and pharmaceutical manufacturing [126]. EPs may be widely dispersed in the environment or left behind in bodies of water.

Among the most prominent are dyes [89], herbicides, pesticides [127,128], phenols [129], PAHs [130,131], pharmaceuticals and personal care products (PPCPs) [132], perfluoroalkyl carboxylates and sulfonates [133]. Due to their tendency to interfere with the function of natural hormones, these compounds have been labeled as endocrine disruptors (EDCs) [134]. They present a high resistance to natural biodegradation and photolysis processes [135]. Toxicity studies of these compounds have associated them with many forms of diseases, such as genotoxicity, developmental and reproductive toxicity, neurotoxicity, and cancer tumors [136].

Most recent studies that have used the HTC process on biowastes to produce adsorbents for EP removal from wastewaters, process conditions, and adsorption performance are summarized in

Table 2. HTC conditions and adsorption performance of HCs for EP removal from water.

Biowaste	HTC Conditions			Adsorption Conditions and Pollutant Retention Efficiency		Ref.
	T (°C)	Residence Time (h)	Activation	Contaminant	Qe (mg/g)/Removal (%)
Rice husks	220	2	-	Diclofenac (DFS) Ciprofloxacin Triclosan (TCS)	>0.002/>83	[137]
Horse manure	220	2	-	Paracetamol Fluconazole Sulfamethoxazole	0.0005/> 49
Glucose + PAN	160	18	NaOH	Paraquat herbicide	437.64/83	[138]
Sugar bagasse	230	24	Fe2+/Fe3+	Tetracycline	48.35/100	[139]
Wood	>180	-	-	Caffeine TCS	8/-	[140]
					5.3/-
Kraft black liquor	170	2	-	Aspirin Paracetamol	50.17/	[141]
					49.95/
Olive mill waste (OMW)	190	6	-	TCS, DFS, Ibuprofen (IBP)	13.7/98 11/64 10/43	[142]
	240				13.8/99 11/73 10/54
Pinewood sawdust/rice husk	300	0.2	Physical	Phenol	83.88/n.a.	[117]
					39.3/n.a.
Fruit powder of Zizipus mauritiana L.	200	20	-	DFS, IBP	2.03 mmol/g/88 2.54 mmol/g/97	[49]
Sludge	160	4	KOH	Bisphenol A (BPA)	10.86/n.a.	[143]
	190				15.11/n.a.
	250				18.37/n.a.

.

From Table 2, it can be observed that the majority of the HCs obtained from different biowastes were not activated after the HTC process, but they presented high removal efficiencies for EPs. Additionally, in the case of composites with poly(acrylonitrile) (PAN), the adsorption capacity showed remarkable value (437.64 mg/g) [138].

In a study performed by Weidemann et al. [137], different biowastes (olive press residues, horse manure, rice husks, and waste from tomatoes) were used as raw materials for preparing low-cost adsorbents via the HTC process for 2 h at 220 °C. The HCs obtained were tested for the removal efficiency of some EPs, such as BPA, ciprofloxacin, DFS, diphenhydramine, fluconazole, octhilinone, paracetamol, sulfamethoxasole, TCS, and trimethoprim. The HCs obtained from rice husks and horse manure HCs exhibited the highest total removal efficiencies, with averages of 66% and 49%, respectively. These values were higher than the corresponding values (32% and 39%) associated with tomato and olive waste HCs, respectively. All four HCs nearly completely removed DCF and BPA. The rice husk HC presented high efficiency (≥83%) for the removal of ciprofloxacin, diphenhydramine, octhilinone, and triclosan. The HC from horse manure was the most effective material for removing fluconazole, sulfamethoxazole, and paracetamol; however, it only removed small amounts of contaminants.

In a comparative study, Kozyatnyk et al. [140] investigated the removal efficiencies of some pharmaceutical compounds for activated carbon (AC) from black charcoal, biochar (BC), and hydrochar (HC). The BC and HC were treated via a thermal process, with the HTC process taking place at temperatures above 180 °C. Based on the data presented in the study, among the three materials, the highest adsorption capacity was that of AC. The average adsorption capacity was high for AC (220 mg/g), but significantly lower for BC and HC (5.5 mg/g and only 4.0 mg/g). This proves how important the activation step is for the final performance of the HC.

In another study, the removal of aspirin and paracetamol from water using bio-based composites made of hydroxymethylated Kraft black liquor and tannin were blended by conventional cross-linking (CCR) and the hydrothermal process (HCR) [141]. The research also showed how two distinct processes—pyrolysis (RFC) and hydrothermal carbonization (RFA)—were used to produce the composites. They reported maximum adsorption capacities of 50.17 mg/g of aspirin adsorbed for RFA and 91.66 mg/g for RFC. For paracetamol, the values obtained were 49.95 mg/g adsorbed for RFA and 73.58 mg/g in the case of RFC. The results demonstrate that the composites can be potential adsorbents for elimination from wastewaters of pollutants with emerging concern.

Delgado-Moreno et al. [142] used an HTC process at 190 and 240 °C in their research to convert HC wastes from the production of olive oil, for the simultaneous removal of DCF, IBP, and TCS from water. The HCs generated from pitted and reprocessed wet olive mill waste (H), olive tree pruning (P), and olive stone (S) were characterized using various methods. The synthetized HCs presented the highest removal efficiency for TCS, 98%, followed by DFC with 64%, and 43% for IBP. This study shows that unmodified HCs from these HCs are affordable, ecologically friendly adsorbents that may be utilized to filter out pharmaceuticals and other similar substances in water.

The removal of DFC and IBP from waters was also investigated by Qureshi et al. [144], who used the HTC process at 200 °C for 20 h on dried fruit powder of Zizipus mauritiana L., resulting in TC-ZM adsorbent. With an adsorption capacity of 2.03 mmol/g for DFS and 2.54 mmol/g for IBP, maximum removal efficiencies of 88% and 97% were achieved, respectively; thus, TC-ZM presents a great potential as an adsorbent for the removal of pharmaceuticals from wastewater.

Mestre et al. [144] synthesized ultra-microporous HCs from sucrose and tested their applicability in removing pharmaceutical compounds from aqueous solutions (caffeine, clofibric acid, ibuprofen, iopamidol, and paracetamol). The performance was observed for iopamidol and paracetamol, with maximum adsorption capacities of 1050 mg/g and 514 mg/g, respectively. These HCs obtained from sucrose showed an increased removal potential when compared with the equivalent commercially available activated carbons (iopamidol, 472 mg/g, and paracetamol, 267 mg/g).

A major difference in adsorption efficiency towards tetracycline was observed using hydrothermal coal obtained by applying the HTC process to Salix psammophila species, when the abundance of acidic groups decreased and the number of basic groups increased accordingly on the adsorbent surface [145]. Tetracycline removal studies were also carried out on magnetic carbon materials (MCMs) obtained from HTC. Tetracycline adsorption appeared to be favored under acidic conditions, with MCM having a positive surface charge [146].

Activated charcoal obtained from HTC-treated orange peels was used as an adsorbent for the removal of flurbiprofen, salicylic acid, and DFS. The best adsorption performance for H₃PO₄-activated charcoal (HC-PN) was for DFS (0.21 mmol/g) and salicylic acid (0.6 mmol/g), as compared with other types of activated charcoal prepared at pH 2 and pH 7, respectively, with different activation methods. HC activated by heat treatment in air (HC-A) presented a better adsorption capacity at pH 2 for flurbiprofen (0.6 mmol/g). Adsorption tests at pH 7 revealed that the DFS is primarily in neutral form under an acidic environment and undergoes a series of reactions leading to deactivation, which further decreases solubility. For salicylic acid and flurbiprofen, pH 2 was considered ideal for the adsorption investigation [147].

3.2. HCs for the Retention of Inorganic Contaminants from Wastewater

It is well recognized that heavy metals such as Hg Zn, Ni, Cd, Cu, Pb, Cr, As, and Se exist in trace amounts in biomass. The study performed by Reza et al. [148] on the heavy metal content in some HCs processed by rice hull, switch grass, miscanthus, and corn stover biowastes at 220, 230, and 260 °C showed the importance of the HTC process for heavy metal removal from raw materials (Figure 4).

Due to the high non-biodegradability, toxicity, and propensity to bioaccumulate in the food chain, the heavy metal pollution of water is recognized as a serious environmental issue, with considerable effects on the environment and living things on a global scale. Some heavy metals frequently linked to water pollution include Cr, Ni, Cd, Zn, and Pb [149].

Table 3. HTC conditions and adsorption performance of HC for heavy metal removal from water.

Biowastes	HTC Conditions			Adsorption Conditions and Pollutant Retention Efficiency		Ref.
	T (°C)	Residence Time (h)	Activation	Contaminant	Qe (mg/g)/Removal (%)
Palm leaves	300	7	-	Pb2+	17.8/n.a.	[150]
			H2O2		74.5/n.a.
Peanut hull	300	5	-	Pb2+	0.88/n.a.	[151]
			H2O2		22.82/n.a.
Sewage sludge	210	5	-	Cu2+	35.8/n.a.	[152]
			KOH		-
			-	o-PO43−	-
			KOH		n.a./97
Tabaco stalk	200	24	Mg and Al	PO43−	41.16/	[153]
Walnut shell powder + polymer	250	1	-	Cr4+	363.22/94.3	[154]
Pine sawdust	180	12	Mg and Si	Cu2+	214.7/99	[94]
				Zn2+	227.3/99
Eucalyptus sawdust	220	0.5	KOH	Cr4+	34.07/85.53	[155]
corn straw					30.15/75.48
corncob					29.46/73.04
Crocus sativus petals	180	11.5	KOH	Pb2+	89.52/-	[156]

presents various studies that have used the HTC process on different biowastes to prepare adsorbents for heavy metal water decontamination applications.

The capacity of HTC-prepared porous carbon materials to absorb and retain heavy metals from aqueous solutions has also been recognized [157,158,159]. In their study, Sun et al. [159] presented a KOH-activated carbon prepared from various feedstock (corn stalks, sawdust, and wheat straws) for the retention of Cd²⁺ and multiple metals (Pb²⁺, Cu²⁺ and Zn²⁺) from water. The HCs were prepared by HTC at 200 °C for 20 h from sawdust, wheat straw, and maize stalk. After the hydrothermal treatments, part of the obtained quantity was modified with KOH; the characterization results showed high contents of C, O, H, and N in the HCs, and surface areas between 4.4 and 9 m²/g. The HCs, unmodified and modified with KOH, were tested for the adsorption of Cd²⁺ from water. Better results were obtained for KOH-activated HCs, with about 80% removal efficiency, compared with unmodified HCs with less than 10%.

Some studies have focused on the removal of Pb²⁺ ions. Rasam et al. [156] prepared a KOH-activated HC from Crocus sativus petals, at a temperature of 180 °C, for 11.5 h, and the adsorbent was tested for its Pb²⁺ adsorption capacity. They found that the materials presented an 89.52 mg/g adsorption capacity. The findings of the experiment show that the temperature and residence time had a significant impact on the final physicochemical characteristics of the generated HC.

In another study [150], Phoenix dactylifera palm leaves (PLs) were treated via the HTC process at 300 °C for 7 h, resulting in HTC-PL materials which were chemically activated with H₂O₂, resulting in AHTC-PL. The removal of Pb²⁺ by HTC-PL HC gave an adsorption capacity of 17.8 mg/g, whereas that of AHTC-PL was much higher: 74.5 mg/g. From the adsorption studies, it was determined that the Pb²⁺ adsorption was best described by a monolayer coverage of the surface (Langmuir isotherm) on the HTC-PL and AHTC-PL.

Another biowaste from agriculture residues was investigated for the removal of lead from waters. Peanut hulls (PHs) [151] were treated using an HTC process for 5 h, at 300 °C (PHHC), which was also modified using H₂O₂ to produce modified (activated) HC (mPHHC). The latter showed enhanced Pb²⁺ adsorption properties, with an adsorption capacity of 22.82 mg/g, which was significantly higher than that of PHHC: 0.88 mg/g. In the same study, a multi-metal system was investigated, and it was found that mPHHC effectively removed Pb²⁺, as well as other heavy metals (Cd²⁺, Cu²⁺, and Ni²⁺) from water. According to the results of these investigations, HCs treated with H₂O₂ could be a useful, affordable, and ecologically friendly adsorbent for use in environmental applications, particularly when it comes to the retention of heavy metals from water.

Another biowaste that may have potential applications in water decontamination is sewage sludge. In a study by Spataru et al. [152], raw and KOH-activated HC, called raw hydrochar (RHC) and enhanced hydrochar (EHC), respectively, were derived from sewage sludge by applying an HTC process at 210 °C, for 5 h. These adsorbents were used for the removal of Cu²⁺ and orthophosphate from wastewater. They found that EHC demonstrated an increased adsorption capacity towards Cu²⁺ and orthophosphate when compared with RHC, presenting an adsorption capacity of 14.3 mg/g and a removal capacity of 97% for the latter. For the removal of phosphate from water, tobacco stalks were hydrothermally treated for 24 h at 200 °C and modified with Mg and Al [153]. The maximum adsorption capacity obtained from the Langmuir model for this adsorbent was 41.16 mg/g at 45 °C.

For the removal of Cr⁶⁺ from water, different biowaste was treated via the HTC process. In one study [154], amino-functionalized HC (AFH) was prepared by the co-processing of walnut shell biowaste and terminal amino hyperbranched polymer in an HTC process at 250 °C, for 1 h, resulting in AFH-50. The adsorbent demonstrated an excellent Cr⁶⁺ adsorption capacity of 363.22 mg/g, suggesting that the co-hydrothermal carbonization provides a homogeneous polymer blending into the HC matrix, thus providing adsorbents for the efficient removal of Cr⁶⁺ from water.

Khushk et al. [155] also studied the removal of Cr⁶⁺ from water. They used corn straws, corncobs, and eucalyptus sawdust as raw materials in an HTC process at 220 °C, for 30 min, with the resulting materials being chemically modified with KOH, resulting in mESD, mCS and mCB adsorbents. The adsorption capacities were 34.07 mg/g, 30.15 mg/g, and 29.46 mg/g, with removal efficiencies of 85.53%, 75.48%, and 73.04% for mESD, mCS, and mCB, respectively.

Different HC composites have also been investigated for the removal of heavy metals from waters. A hybrid silicate-HC composite (MgSi-HC) was synthesized in an HTC process at 180 °C for 12 h and tested for the removal of Cu²⁺ and Zn²⁺ from water [94]. The maximum adsorption capacities of MgSi-HC were 214.7 mg/g (Cu²⁺) and 227.3 mg/g (Zn²⁺), and the removal efficiencies were over 99% for both metal ions.

Han et al. [160] investigated the possibility of using HC as an adsorbent for Cd²⁺ removal from water. Adsorbents were prepared from poultry litter and pig solids via the HTC process at 250 °C, and they were used for the removal of Cd²⁺ and Sb³⁺. The maximum adsorption capacities for Cd²⁺ were 19.80 mg/g and 27.18 mg/g for HC from poultry litter and pig solids, respectively. For Sb³⁺, the adsorption capacity was lower, with a maximum of 3.98 mg/g for HC produced from pig solids.

All the studies suggest that HCs derived from biowastes via the HTC process can be effective, low-cost adsorbents for heavy metal removal from waters, but they present better results when an activation step is applied after, or even during the HTC process.

4. Challenges

Although a number of advantages of the HTC process have been identified as a technology to recover biomass from various sources, the technique has drawbacks that preclude industrial implementation. The main disadvantage is that, because of the high amount of dissolved mineral and organic chemicals, post-process water cannot be used as a liquid fertilizer without pre-treatment [161], and the cost associated with such an approach is an economic disadvantage [162,163].

Even though the HTC process is a promising method for converting wet biowaste into valuable materials, the nature of the resulting products (HCs) is largely influenced by the reaction conditions of the process, particularly the residence time and the temperature used [164]. Therefore, optimization of these parameters must be considered for the synthesis of HCs with enhanced adsorption properties, which can make the process laborious, expensive, and time consuming. Even though the HTC process requires a lower energy consumption than other thermochemical processes, the 180–250 °C range is still considered to be economically expensive [165].

The HTC process attracted great interest in green society development. The process has being studied for over 100 years; the construction of the first commercial plant was in 2016 by TerraNova Energy [166]. Based on this successful experience, in 2021, the largest HTC organic waste processing plant in the world was opened. Even if the up-scaling process was achieved, the cost–benefit analysis is still being developed, combined with environmental issues.

There are still a few gaps regarding the reliability and efficiency of the process, especially related to the mechanism of HC formation and the possibility of liquid phase organic contamination due to their waste sources being used as raw material. Factors such as pressure, reaction mechanism, and kinetics are still being studied and reviewed [10].

Potential applications of HC depend on the processing step and precursor materials, especially when manure or sludge are used as carbon sources. Electrospinning or 3D printing techniques could yield smart architectural carbonaceous structures with photocatalysis, adsorbent, biosensors, and optoelectronic properties.

A challenge is represented by the separation of different compounds from the raw materials, especially from biomass wastes, when the goal is obtaining advanced material designs. As an environmental tool, HTC could represent a reliable and sustainable approach; however, the data are almost non-existent.

Perhaps the most important challenge for the HTC process is the efficiency regarding the degradation of emerging organic contaminants present in wastes. There are a few studies regarding the degradation mechanisms of organic compounds from precursor materials. If results indicate HTC performance for pathogen-free HC [167], future investigations regarding persistent emerging organics, such as compositions of wastes used for the HTC process, have to be developed.

Based on recent studies, composites prepared from HC and other compounds can be good potential materials for the removal of emerging pollutants from wastewater. Composites based on magnetite and HC derived from sunflower husk were prepared and analyzed. These could potentially be utilized as adsorbents for environmental remediation, particularly as an adsorbent of emergent contaminants from wastewater [168]. Another study reported preparing an HC/Ag₃PO₄ composite and using it as an efficient photocatalyst for the degradation of sulfamethoxazole [169]. Composites based on HC and TiO₂ were reported in a study by Mengting et al., in which HC was obtained from wheat straws by the HTC process at 180 °C for 6 h [170]. The surface was functionalized with different amounts of TiO₂ via a sol–gel method, and the final product was tested for the removal of tetracycline adsorption from aqueous solutions with a maximum removal efficiency of 93%. New multifunctional carbon materials, such as magnetic carbonaceous nanocomposite (MCN), synthesized by the HTC process in the presence of carbohydrate and Fe³⁺, could be an adsorbent starting point for many kinds of biomass [171].

5. Conclusions

Although many studies have focused on the optimization of HTC process parameters for the valorization of biowaste to valuable chemicals and materials, there are not many studies that have focuses on the potential water decontamination applications of hydrochar derived from biowaste via the HTC process.

The HTC process offers an economical, efficient, and emerging way to valorize biowaste which is otherwise difficult and costly to manage, store and recover. This technique is one of the most widely used approaches when it comes to converting wastes into valuable products, due to the high flexibility of the process parameters (i.e., temperature and reaction time), and the ease of using various catalysts to enhance the generation of valuable chemicals in the liquid phase and/or the HC yield. These HCs can have applications in many fields, such as environmental remediation, energy production, water treatment, and soil conditioning.

Products generated in HTC processes can be tailored to industry necessities by applying additional post-treatments, enabling industries to replace fossil coal, thereby diminishing CO₂ emissions and supporting the circular bioeconomy.

The HTC process is performed in an aqueous environment; therefore, it does not require a drying step for the use of wet biomass as raw material. Among the most common biowastes used in this process are those produced by agro- and food industries, municipalities (solid waste and sewage sludge), as well as the forestry and papermaking industries.

HC from biowaste can be used as an efficient adsorbent of different types of pollutants from wastewater. Although these materials have low porosities, due to their functional surface, they can increase their adsorption selectivity, and some promising results have been obtained for pollutants such as dyes, heavy metals, and pharmaceutical products. In addition, chemical activation combined with physical activation can improve performance due to pore development.

Further experiments should focus on optimizing the preparation and activation of HC for specific needs, as well as examining the regeneration and reutilization abilities in a circular bioeconomy context.