Hydrothermal Carbonization Technology for Wastewater Treatment under the “Dual Carbon” Goals: Current Status, Trends, and Challenges

: Hydrothermal carbonization (HTC) technology transforms organic biomass components, such as cellulose and lignin, into valuable carbon materials, gases and inorganic salts through hydrolysis, degradation and polymerization, with significant advantages over traditional methods by reducing energy consumption, lowering pollutant emissions and enhancing carbonization efficiency. In the context of global climate change, HTC plays a critical role in water environment management by addressing industrial, agricultural, and domestic wastewater challenges. The application of HTC extends to wastewater treatment, where hydrochar effectively adsorbs heavy metals, organic compounds, and anions, thereby improving water quality. However, challenges remain, such as optimizing the process for diverse raw materials, managing economic costs, and addressing environmental and social impacts. Future research and policy support are essential for advancing HTC technology. By enhancing reaction mechanisms, developing catalysts, and promoting international cooperation, HTC can significantly contribute towards achieving carbon neutrality goals and fostering sustainable development.

In general, (hydro)thermal techniques can be divided into two main categories: thermal treatments (operating under inert atmosphere and dry conditions) and hydrothermal treatments (operating in a closed pressurized system and under wet conditions).Each category can be further divided into carbonization, liquefaction, and gasification treatments, based on the operating temperature and phase partitioning of the products (Figure 1).In this paper, the hydrothermal carbonization treatment will be mainly introduced [23].
category can be further divided into carbonization, liquefaction, and gasification treatments, based on the operating temperature and phase partitioning of the products (Figure 1).In this paper, the hydrothermal carbonization treatment will be mainly introduced [23].Hydrothermal carbonization (HTC), a recently prominent carbonization technology, is increasingly recognized for its applications in environmental protection [21,24].This process utilizes water as a thermal medium in a sealed high-temperature, high-pressure environment to transform waste into useful substances such as carbon materials, gases, and inorganic salts [25][26][27].Compared to traditional carbonization methods, HTC offers significant advantages, including lower energy consumption, reduced pollutant emissions, higher carbonization efficiency, and greater value-added products [22,27].
As urban modernization progresses rapidly, living standards in cities have significantly improved.Concurrently, China's water environments are increasingly stressed.The discharge of industrial, agricultural, and domestic wastewater has progressively increased, exacerbating water pollution [28][29][30].In addition, the development of the pharmaceutical industry, animal husbandry and the unreasonable discharge of wastewater, is causing antibiotic pollution and it is particularly prominent in the water environment [31].The "China Ecological Environment Statistical Yearbook (2018-2023)" published by the Ministry of Ecology and Environment reports high pollutant levels in discharged wastewater, heavily impacting water environments.Industrial and agricultural wastewater discharges have significantly polluted rivers and lakes, harming the ecosystem [19,20,28,32,33].Consequently, effectively addressing water environmental issues has become an urgent problem. Figure 2 is a statistical chart of the number of papers on the application of hydrochar in wastewater treatment from 2014 to 2024 in June.In 2014-2019, the number of papers in this area is small, and the number of papers after 2020 increases significantly, indicating that in recent years, people have conducted more and more research on the application of hydrochar for wastewater treatment, which reflects the development of this technology, the increasing attention to water environment problems, and the acceleration of the practical application and transformation of research results.In Figure 3, it can see the main keywords used in articles on applications and use the search term, on the scientific network.The terms ʺhydrochar yieldʺ, ʺhigher heating valueʺ, Hydrothermal carbonization (HTC), a recently prominent carbonization technology, is increasingly recognized for its applications in environmental protection [21,24].This process utilizes water as a thermal medium in a sealed high-temperature, high-pressure environment to transform waste into useful substances such as carbon materials, gases, and inorganic salts [25][26][27].Compared to traditional carbonization methods, HTC offers significant advantages, including lower energy consumption, reduced pollutant emissions, higher carbonization efficiency, and greater value-added products [22,27].
As urban modernization progresses rapidly, living standards in cities have significantly improved.Concurrently, China's water environments are increasingly stressed.The discharge of industrial, agricultural, and domestic wastewater has progressively increased, exacerbating water pollution [28][29][30].In addition, the development of the pharmaceutical industry, animal husbandry and the unreasonable discharge of wastewater, is causing antibiotic pollution and it is particularly prominent in the water environment [31].The "China Ecological Environment Statistical Yearbook (2018-2023)" published by the Ministry of Ecology and Environment reports high pollutant levels in discharged wastewater, heavily impacting water environments.Industrial and agricultural wastewater discharges have significantly polluted rivers and lakes, harming the ecosystem [19,20,28,32,33].Consequently, effectively addressing water environmental issues has become an urgent problem. Figure 2 is a statistical chart of the number of papers on the application of hydrochar in wastewater treatment from 2014 to 2024 in June.In 2014-2019, the number of papers in this area is small, and the number of papers after 2020 increases significantly, indicating that in recent years, people have conducted more and more research on the application of hydrochar for wastewater treatment, which reflects the development of this technology, the increasing attention to water environment problems, and the acceleration of the practical application and transformation of research results.In Figure 3, it can see the main keywords used in articles on applications and use the search term, on the scientific network.The terms "hydrochar yield", "higher heating value", "mechanism", "adsorption" and "functional group" are more controllable terms, which may indicate that the adsorption application of hydrochar is related to the mechanism of hydrochar, the type of functional groups, and the yield and calorific value of hydrochar also have indispensable value for its application.
Water 2024, 16, 1749 3 of 28 ʺmechanismʺ, ʺadsorptionʺ and ʺfunctional groupʺ are more controllable terms, which may indicate that the adsorption application of hydrochar is related to the mechanism of hydrochar, the type of functional groups, and the yield and calorific value of hydrochar also have indispensable value for its application.In response to global climate challenges and to achieve the "Dual Carbon" goals of peaking and neutralizing carbon emissions, continuous innovation in wastewater management technologies is essential.HTC, as an emerging treatment technology, offers significant advantages and broad application prospects.This paper explores the current applications of HTC in wastewater treatment, including its effectiveness against various pollutants, technological processes, and engineering examples.We also analyze the principles and historical development of this technology, addressing current challenges, research directions, and policy recommendations.Through this study, we aim to advance the development and application of HTC, thereby significantly contributing to water environment management and protection.We believe that with ongoing technological advancements and policy improvements, HTC will play a crucial role in achieving the "Dual Carbon" goals and promoting sustainable development.

Basic Principle
HTC is a process that converts biomass into carbon-rich materials using a hightemperature, high-pressure aqueous environment [24,26,27].This technology operates in the absence of oxygen, preventing combustion and oxidation, thus preserving the carbon content of the raw materials.It not only transforms waste into valuable resources but also reduces environmental pollution.The carbon materials produced take the advantages of low cost, excellent chemical stability, large surface area and abundant pores [34].
HTC is a thermochemical conversion technique that processes biomass with water at specific temperatures (180-250 • C) and pressures (2-10 MPa) into high-value carbon products [32,35], along with significant liquid and minor gaseous byproducts, primarily CO 2 [36,37].This technology effectively converts fibrous materials found in animal manure and crop residues, including cellulose, hemicellulose, and lignin [27].HTC involves the decomposition of macromolecules into smaller molecules, which then re-polymerize [37].The process includes hydrolysis, dehydration, decarboxylation, condensation, and aromatization [38,39].Cellulose, for example, undergoes hydrolysis where C-O and C-C bonds break, leading to the formation of intermediates like 5-hydroxymethylfurfural, followed by dehydration and aromatization to form hydrophobic microsphere cores and hydrophilic shells through aldol condensation [40].Similarly, hemicellulose and lignin undergo specific breakdown and reformation processes to produce high-quality, high energy-dense hydrochar [41,42].HTC's strength lies in the precise control of reaction parameters (temperature, pressure, feedstock particle size, and duration) enabling the production of superior quality carbons with wider application potential [43,44].In Figure 4, the main mechanism of hydrochar formation is highlighted by describing the HTC reaction pathways from lignocellulosic biomass.HTC traces its origins to the early 19th century when the formation mechanisms of coal began to be studied.In Figure 5, the history of HTC technology is described.In 1913, the German chemist Friedrich Bergius and his colleagues conducted carbonization of cellulose under hydrothermal conditions at temperatures ranging from 250 to 310 °C, resulting in a black carbonaceous material with a significantly reduced oxygen-to-carbon (O/C) atomic ratio compared to the raw material.Subsequently, researchers expanded the range of feedstocks to other biomass materials and systematically studied HTC processes.
In the 1970s, HTC technology began to garner attention from scientists and researchers, who explored the transformation of biomass into carbonaceous products using hightemperature, high-pressure water.These studies primarily focused on the thermal decom- HTC traces its origins to the early 19th century when the formation mechanisms of coal began to be studied.In Figure 5, the history of HTC technology is described.In 1913, the German chemist Friedrich Bergius and his colleagues conducted carbonization of cellulose under hydrothermal conditions at temperatures ranging from 250 to 310 • C, resulting in a black carbonaceous material with a significantly reduced oxygen-to-carbon

Main Types and Methods of Hydrochar
HTC, as a green and sustainable method for carbon material production, has garnered extensive attention and research in China.Based on various process conditions and operational principles, this technology can be categorized into the following main types: 2.2.1.Ordinary HTC In a closed system, biomass is mixed with water and carbonized under specific temperature and pressure conditions, resulting in a product that is black, solid, rich in oxygenated functional groups, and primarily composed of carbon.The standard HTC process is relatively simple and efficiently utilizes biomass resources.

HTC Using Catalysts
During the resource recovery process from waste, optimizing energy consumption is essential to accommodate variations in biomass feedstocks and enhance the yield and quality of hydrochar [47].In the reaction, the rate of biomass dehydration and carbon product formation is generally low, particularly when using plant biomass, which results in hydrochar with lower carbon content, surface area, porosity, and thermal stability, limiting its practical applications [48].Introducing catalysts can improve the efficiency and quality of the carbonization process.Catalysts facilitate the degradation and carbonization of organic materials in biomass, yielding carbon materials with more complex structures and superior properties.Common catalysts include salts, acids, metal oxides, molecular sieves, and hydroxides [47].In Table 1, some studies are described to better express the effect of using catalysts on HTC technology.
Petrovic et al. [49] utilized grape residue to prepare hydrochar by KOH modification under the condition of 220℃ and 1 h of hydrothermal temperature.Obtained results showed that the KOH treatment increased the sorption capacity of hydrochar from 27.8 mg/g up to 137 mg/g at pH 5. Adsorption of lead on either of the materials was achieved through the ion-exchange mechanism, chemisorption and Pb 2+ -π interaction.The Sips isotherm model gave the best fit with the experimental data obtained for Pb 2+ sorption using activated hydrochar.Similarly, Jiang et al. [50] prepared hydrochar by using Sedum Alfredii Hance as raw material under KOH modification at 180-270 °C for 5 h.Research showed that the adsorption capacity of hydrochar for Cd(Ⅱ) was greatly enhanced after KOH modification, due to improved specific surface areas and pore structure.The maximum Cd(II) adsorption capacity of modified hydrochar was 25.69 mg/g.Additionally, Qian et al. [51] utilized bamboo sawdust as raw material to prepare hydrochar maintained at 473 K for 24 h and treated with a NaOH modification.The adsorption capacity of hydrochar to methylene blue after the modified treatment is 655.76 mg/g.The high adsorption capacity In the 1970s, HTC technology began to garner attention from scientists and researchers, who explored the transformation of biomass into carbonaceous products using hightemperature, high-pressure water.These studies primarily focused on the thermal decomposition reactions of lignocellulosic biomass [46].
In the 1990s, HTC technology was increasingly applied to waste management and energy conversion.Researchers conducted successful studies on various types of biomasses, including food waste, agricultural residues, and other discarded plant materials, achieving promising results.Since the 21st century, with the increasing prominence of energy and environmental concerns, HTC has emerged as a viable method for waste management and energy conversion.As technology has advanced, there has been a deeper understanding and research into its applications and the value of its products.

Main Types and Methods of Hydrochar
HTC, as a green and sustainable method for carbon material production, has garnered extensive attention and research in China.Based on various process conditions and operational principles, this technology can be categorized into the following main types: 2.2.1.Ordinary HTC In a closed system, biomass is mixed with water and carbonized under specific temperature and pressure conditions, resulting in a product that is black, solid, rich in oxygenated functional groups, and primarily composed of carbon.The standard HTC process is relatively simple and efficiently utilizes biomass resources.

HTC Using Catalysts
During the resource recovery process from waste, optimizing energy consumption is essential to accommodate variations in biomass feedstocks and enhance the yield and quality of hydrochar [47].In the reaction, the rate of biomass dehydration and carbon product formation is generally low, particularly when using plant biomass, which results in hydrochar with lower carbon content, surface area, porosity, and thermal stability, limiting its practical applications [48].Introducing catalysts can improve the efficiency and quality of the carbonization process.Catalysts facilitate the degradation and carbonization of organic materials in biomass, yielding carbon materials with more complex structures and superior properties.Common catalysts include salts, acids, metal oxides, molecular sieves, and hydroxides [47].In Table 1, some studies are described to better express the effect of using catalysts on HTC technology.
Petrovic et al. [49] utilized grape residue to prepare hydrochar by KOH modification under the condition of 220°C and 1 h of hydrothermal temperature.Obtained results showed that the KOH treatment increased the sorption capacity of hydrochar from 27.8 mg/g up to 137 mg/g at pH 5. Adsorption of lead on either of the materials was achieved through the ion-exchange mechanism, chemisorption and Pb 2+ -π interaction.The Sips isotherm model gave the best fit with the experimental data obtained for Pb 2+ sorption using activated hydrochar.Similarly, Jiang et al. [50] prepared hydrochar by using Sedum Alfredii Hance as raw material under KOH modification at 180-270 • C for 5 h.Research showed that the adsorption capacity of hydrochar for Cd(II) was greatly enhanced after KOH modification, due to improved specific surface areas and pore structure.The maximum Cd(II) adsorption capacity of modified hydrochar was 25.69 mg/g.Additionally, Qian et al. [51] utilized bamboo sawdust as raw material to prepare hydrochar maintained at 473 K for 24 h and treated with a NaOH modification.The adsorption capacity of hydrochar to methylene blue after the modified treatment is 655.76 mg/g.The high adsorption capacity of hydrochar for methylene blue indicates that alkaline treatment followed by hydrothermal carbonization in acidic media has potential applications for producing highly efficient MB adsorbents for wastewater treatment.
Huang et al. [52] utilized sludge as raw material under hydrothermal conditions at 180 • C for 2 h, with a citric acid dosage ratio (citric acid to dry sludge mass ratio) of 0.1.The produced hydrochar featured a maximum surface area of 59.95 m 2 /g, rich in oxygencontaining functional groups, and exhibited the highest equilibrium adsorption capacity for Pb(II).Wang et al. [53] activated hydrochar produced from sawdust at temperatures between 200 and 250 • C and a heating rate of 4 • C/min using acetic acid, which reduced the energy consumption of the HTC reaction.The resultant hydrochar had a higher calorific value and energy density, making it suitable for biofuel applications.Chen et al. [54] used bamboo powder as raw material and activated the product in citric acid solution under hydrothermal conditions at 180 • C for 6 h.Citric acid is an efficient HTC additive to improve the carbonization degree, to optimize the pore structure distribution, and to introduce more oxygen-containing functional groups (OFGs) to hydrochar, which offer an appropriate precursor to be activated by KOH to improve the specific surface area (SSA) level and electrochemical performance of the activated porous carbon.
Huang et al. [55] utilized raw sewage sludge as raw material and modified hydrochar with calcium acetate (CaAc 2 ) and sodium acetate (NaAc) at 160-250 • C for 30 min.This modification increased the surface area and pore volume compared to unmodified samples and achieved a phenol removal rate of 65.7%.The addition of Ca acetate reduces the nitrogen retention in the hydrochar due to enhanced protein hydrolysis and deamination, leading to more nitrogen transformation into the NH 4 + -N in the aqueous product.The addition of Na acetate also slightly enhances protein hydrolysis, thus increasing the polypeptide-N in the aqueous product.Mumme et al. [56] conducted HTC of cow dung and corn digestate at 190-270 • C using natural zeolite as a catalyst.Adding zeolite enhances the carbon, hydrogen, and ash content, improves energy recovery rates, increases the surface area and pore volume of the hydrochar, and significantly boosts the recovery rates of nitrogen and sulfur.Lang et al. [57] activated hydrochar from pig manure with CaO at temperatures of 180, 200, and 220 • C for 10 h, and significantly increased the pH and yield of the hydrochar, although the recovery rates of carbon and nitrogen were slightly decreased.Additionally, the addition of CaO facilitates the transformation of phosphorus from non-apatite inorganic phosphorus (NAIP) to apatite phosphorus (AP), with nearly all the phosphorus in the pig manure recovered as AP in the hydrochar through CaO-assisted HTC.The addition of calcium acetate can reduce the retention of nitrogen in the hydrate, and the addition of sodium acetate can slightly enhance the hydrolysis of the protein.Increases the surface area and pore volume of the hydrochar, and significantly boosts the recovery rates of nitrogen, sulfur and energy. [56] Pig manure 180, 200, and 220 • C, 10 h metal oxides CaO Facilitates the transformation of phosphorus from non-apatite inorganic phosphorus (NAIP) to apatite phosphorus (AP). [57]

Multi-Step HTC
During the HTC process, raw materials are mixed with a suitable solvent such as water in a sealed container.The reaction conditions are manipulated through multiple stages of varying temperatures and durations to produce hydrochar with unique porous structures and surface characteristics, enhancing its utility across various fields.Specifically, the two-step method involves an initial stage under low temperatures and high pressure to degrade organic matter in biomass into smaller organic molecules.The second stage operates at high temperatures and lower pressure to further carbonize these molecules into carbon materials.This multi-step approach enhances carbonization efficiency, optimizes the structure of the carbon materials, and reduces energy loss.
Zhang et al. [58] synthesized a novel arsenic adsorption material, iron-modified hydrochar, by using different iron species, i.e., FeCl 3 •6H 2 O, FeSO 4 •7H 2 O, and Fe(NO 3 ) 3 •9H 2 O.These hydrochars were prepared through a one-step hydrothermal carbonization process at 220 • C. The results indicated that compared with FeCl 3 •6H 2 O and FeSO 4 •7H 2 O, hy- drochar modified with Fe(NO 3 )3•9H 2 O exhibited a maximum iron retention rate of 84.2% and a maximum arsenic adsorption capacity of 11.19 mg/g.Jiao et al. [59] proposed an effective strategy for synergistic production of high value-added xylooligosaccharides (XOS) and humic-like acid (HLA) from vinegar residue based on a two-step hydrothermal pretreatment.During the first-step hydrothermal pretreatment (170 & DEG; C, 50 min), 29.1% of XOS (X-2-X-6) was obtained.The XOS yield was further improved to 36.2% with endoxylanase hydrolysis, thereby increasing the value of (X-2-X-4)/XOS from 0.8 to 1.0.Subsequently, the second-step hydrothermal pretreatment was investigated to produce Water 2024, 16, 1749 8 of 28 HLA from the solid residue of the first-step hydrothermal pretreatment.The highest HLA yield was 15.3% in the presence of 0.6 mol/L of KOH at 210 • C for 13 h.In addition, 31.7% of hydrochar by-product was obtained.The mass balance results showed that 1000 g of vinegar residue produced 67.9 g of XOS, 91.6 g of HLA, and 189.5 g of hydrochar.Tan et al. [60] presented a novel approach to synthesize nitrogen-doped porous carbon materials via a three-step fabrication process using citric acid as the carbon source and urea as the nitrogen source.Firstly, hydrochar was synthesized by a microwave-assisted hydrothermal method using citric acid and urea and as the reactants.The hydrochar was then subjected to hightemperature carbonization in an Ar atmosphere followed by KOH activation, resulting in nitrogen-doped porous carbon materials.The as-prepared porous carbon possesses a high BET surface area of 2397 m 2 /g and an average pore size of 1.8 nm.Such N-rich porous carbon shows outstanding capacitive performance (365 F/g at 0.5 A/g), good rate capacitive behavior, and excellent cycling stability, indicating a great potential for supercapacitors.

HTC Used in the Hydrothermal Process of Liquid Circulation
During the HTC process, the liquid by-products are recycled to enhance the biomass conversion rate and the production efficiency of carbon materials.The recycling of the process liquid significantly improves the resource recovery efficiency of the hydrothermal fluid, maximizing the value of the products and enhancing water resource utilization.The use of hydrothermal fluid as a recyclable medium not only reduces water consumption, but also offers benefits such as the reuse of thermal energy, increasing the efficiency of energy recovery, and reducing the costs of HTC processing [61].Furthermore, recycling hydrothermal fluid significantly reduces the volume of wastewater produced, decreases the costs of wastewater treatment and contributes to environmental protection [62].
Stemann et al. [62] conducted 19 cycles of process water under hydrothermal conditions at 220 • C for 4 h.During the first five cycles, the total organic carbon (TOC) concentration was increased, and then stabilized.The accumulation of organic acids in the liquid phase enhances the calorific value and carbon concentration of the solids, thereby increases energy density.Uddin et al. [63] used Pinus taeda at hydrothermal temperatures of 200-260 • C with a residence time of 5 min and a water-to-biomass dry basis mass ratio of 5.They recycled the process at 200 and 230 • C for 9 cycles and 260 • C for 5 cycles.The biomass carbon yield was 5-10% higher than the initial cycle at each investigated temperature, with the higher heating value (HHV) remaining essentially unchanged.The TOC concentration in the aqueous phase was gradually concentrated with each cycle, reaching equilibrium.

Co-HTC
Co-HTC involves mixing two or more types of biomasses and processing them together.This method leverages the differences in properties and densities of the materials, resulting in a synergistic effect that significantly enhances combustion.This synergy allows the mixture to burn more completely, releasing greater amounts of heat and making more efficient use of waste biomass materials [47,64,65].Additionally, co-carbonization can lower the carbonization temperature required for individual materials, thus improving resource utilization.The cotreatment extends the temperature interval between hydrolysis and pyrolysis, reducing the activation energy required for pyrolysis.Furthermore, it significantly improves ignition and combustion characteristics, shifting the peak combustion rate and its temperature range to higher temperatures, thereby lowering the activation energy of the combustion reaction [47].In Table 2, several research are carried out to better understand co-HTC technology.
Zhu et al. [64] mixed food waste digestate (FWD) with wood waste (WW) as feedstock for co-HTC at 250 • C for 2 h.The results showed that the combination strategy enhanced the fuel properties of the hydrochar.In particular, the ash content of hydrochar was reduced to a minimum of 6.3%, thereby increasing the heating value by nearly two folds.The comprehensive combustion and combustion stability indices were improved with the maximum values of 3.98 × 10 −9 % 2• C −5 and 4.22 × 10 2 % • C −3 , respectively.Wang et al. [65] used food waste (FW) and woody sawdust (WS) as feedstock for conduct-ing co-HTC at 180-260 • C for 1 h with a stirrer speed of 100 r/min.Results suggested that hydrochar yield consistently decreased with an increase in both the FW ratio and HTC temperature.The C retention from 260 • C hydrochar was low (approximately 65%), but more microsphere structures were formed due to the enhanced carbonization degree of the hydrochar.Li et al. [66] mixed livestock manure (SM) with corn cob (CC) as feedstock for co-HTC at 180-260 • C for 2 h.Compared with HTC of SM, the addition of CC could effectively reduce the ash content, enhance the N recovery to 38.95-47.61%(SM:CC = 1:1), and increase the surface porous structure, making the Co-hydrochars suitable as fertilizers.Under the optimal hydrothermal conditions of 240 • C, 2 h, and mixing ratio of 1:1, the hydrochar yield was as high as 36.72%, and the TNC (6.341%), N recovery rate (47.61%) and P recovery rate (86.41%) were all suitable for fertilizer.Sharma and Dubey [67] mixed food waste (FW) and yard waste (YW) for co-HTC at 220-260 • C for 1-4 h.They found that the calorific value of blended raw feedstock was 13.5 MJ/kg which increased to 27.6 MJ/kg after Co-HTC at 220 • C for 1 h.The energy yield and fuel ratio calculated was 45% and 0.65, respectively.The hydrochar produced demonstrated a stable combustion profile as compared to reactive combustion profile for raw samples.Shen et al. [68] used corn straw and chlorella as raw materials for co-HTC at 240 • C for 1 h.They observed that moderate reaction conditions favored nitrogen enrichment and increased porosity in the hydrochar.Under these conditions, the optimal nitrogen content and surface area of the product were 3.50% and 5.91 m 2 /g, respectively.

HTC at Different Temperatures
In the HTC process, the temperature significantly impacts the properties and structure of the carbonized products.Variations in hydrothermal temperatures yield diverse products, and studying these differences is crucial for developing high-performance carbon materials for practical applications.Higher temperatures (above 300 • C) facilitate the effective degradation of organic materials in biomass, resulting in high-purity carbon materials with greater potential application value.Conversely, lower temperatures (below 300 • C) are more conducive to preserving carbon.In Table 3, some studies were performed to better understand the effects of different temperatures on HTC technology.
Oxidized hydrothermal biochar was prepared by hydrothermal carbonization of Spartina alterniflora biomass (240 • C for 4 h) and subsequent oxidization (240 • C for 10 min) under air.Oxidized hydrochar achieved a Fe(III) reducing capacity of 2.15 mmol/g at pH 2.0 with 120 h, which is 1.2 times higher than un-oxidized hydrochar.Low temperature oxidization increases the contents of carboxyl and carbonyl groups on the hydrochar surface.
This study reveals that low temperature oxidization is an effective way to improve and restore the abiotic reducing ability of hydrochar [69].The hydrochar yield and carbon retention in processing the livestock manure were decreased when increasing the reaction temperatures between 180 and 240 • C for 1 h [70].Nguyen et al. [71] performed the reaction with grape marc at 180-260 • C for 30 min to prepare the hydrochar slurries for ignition experiments.It has shown that the 260 • C solid hydrochar exhibited the shortest ignition delay time (0.2 s) and the lowest ignition temperature (179 • C).
Gao et al. [72] used eucalyptus bark as the raw material in the reaction for 2-10 h in the range of 220-300 • C to obtain the hydrochar.With the increase in temperature, the yield of hydrochar decreased slightly from 46.4% at 220 • C to 40.0% at 300 • C. The atomic ratio of O/C and H/C decreased from 1.69 and 0.80 to 0.83 and 0.23, respectively, and the oxygen-containing functional groups decreased with increasing temperature.Peng et al. [73] used glucose as the raw material and kept it at 160-220 • C for 1-12 h to make the hydrochar.The experimental study showed that the adhesion of hydrochar was different: the lower the temperature, the greater the adhesion.Hydrochar is mainly composed of furan domains and aromatic clusters, and its surface is rich in oxygen-containing functional groups.The increase in the reaction temperature (160-220 • C) enhanced the aromatization degree of the hydrochar.Li et al. [74] prepared hydrochar with chicken, dairy, and swine manures at 200-350 • C for 2 h.The results showed that high temperature resulted in low yield and decreased H/C, O/C and volatiles of hydrochar.The high temperature results in a higher fixed carbon, fuel ratio and calorific value of the hydrochar, which indicates that hydrochar from animal manures can be used as a substitute for fuel.The carbon retention rate decreases with the increase in temperature.High temperature improves the aromatics of hydrochar and enhances its thermal oxidation resistance.With the increase in temperature, the carbon retention rate decreased, and the aromatics and thermal oxidation resistance is increased.[74] In practical applications, the choice of carbonization methods and operational conditions varies based on specific needs and circumstances.For example, in waste management, HTC can transform waste into hydrochar and steam, extracting valuable substances and energy.In the field of biomass energy conversion, this technology can process biomass into high-purity, high energy-dense hydrochar, which can be further utilized in the production of fuels, chemicals, and electrode materials.In Table 4, the advantages of the methods for the different types of HTC technology are described.
In HTC, the conversion of biomass into carbon materials involves several critical steps.In Figure 6, the general steps of HTC technology are described.Initially, biomass constituents such as cellulose, sugars, and lignin are mixed with water to create a slurry.This mixture is then transferred to a sealed reactor or autoclave, where it is heated to a predetermined temperature for a specific duration.Under these high temperature and pressure conditions, the biomass undergoes a series of chemical reactions including hydrolysis, degradation, and polymerization, leading to the formation of carbon materials.Once the reaction is completed, the reactor is cooled to room temperature, and the solid carbonaceous product is removed, washed, and dried.Finally, to enhance the properties of the carbon material, further post-treatment steps such as activation or surface modification may be employed.

Type
Methods Advantage

HTC
Biomass is mixed with water and directly carbonized under specific temperature and pressure conditions.
Makes full use of biomass resources, less process, and low difficulty to operate.
HTC with catalyst A certain amount of catalyst was added to modify the hydrochar.
Complex structure and excellent performance, which improves the HTC yield and energy recovery efficiency.

Multi-step HTC
The reaction conditions are manipulated through multiple stages of varying temperatures and durations.
Has unique pore structure and surface characteristics, which optimizes the structure, improves the carbonization efficiency and reduces the energy loss.

Liquid phase circulating HTC
Repeatedly recycle by-products of hydrothermal fluid.
Improves the conversion rate of biomass and utilization efficiency of water resources and reduces the cost of carbonization process and wastewater treatment.
Co-HTC Mix a variety of raw materials together and carry out HTC at the same time.
Reduces the carbonization temperature, and improves the material performance, combustion performance and resource utilization rate.

High temperature HTC
The hydrothermal temperature is higher in the carbonization process.
High calorific value, and excellent product stability, with long-term carbon sequestration potential.

Low temperature HTC
The hydrothermal temperature is low in the carbonization process.
Has better hydrochar yield, carbon recovery and energy recovery, and has an obvious short-term carbon sequestration effect.Improves the conversion rate of biomass and utilization efficiency of water resources and reduces the cost of carbonization process and wastewater treatment.
Co-HTC Mix a variety of raw materials together and carry out HTC at the same time.
Reduces the carbonization temperature, and improves the material performance, combustion performance and resource utilization rate.High temperature HTC The hydrothermal temperature is higher in the carbonization process.
High calorific value, and excellent product stability, with long-term carbon sequestration potential.
Low temperature HTC The hydrothermal temperature is low in the carbonization process.
Has better hydrochar yield, carbon recovery and energy recovery, and has an obvious short-term carbon sequestration effect.
In HTC, the conversion of biomass into carbon materials involves several critical steps.In Figure 6, the general steps of HTC technology are described.Initially, biomass constituents such as cellulose, sugars, and lignin are mixed with water to create a slurry.This mixture is then transferred to a sealed reactor or autoclave, where it is heated to a predetermined temperature for a specific duration.Under these high temperature and pressure conditions, the biomass undergoes a series of chemical reactions including hydrolysis, degradation, and polymerization, leading to the formation of carbon materials.Once the reaction is completed, the reactor is cooled to room temperature, and the solid carbonaceous product is removed, washed, and dried.Finally, to enhance the properties of the carbon material, further post-treatment steps such as activation or surface modification may be employed.

Application of HTC in Water Environment Management
Water environment management is essential for global sustainable development [29,30,33].HTC technology represents an emerging environmentally friendly approach for revolutionizing global water environmental management.Primarily, this technology leverages the heat and pressure conditions in aqueous environments to transform organic materials into carbon materials with outstanding properties.As a novel green technology, HTC holds broad prospects for application in water environmental governance.This paper will explore in detail the applications of HTC in managing water environments, including wastewater treatment, surface water restoration, groundwater purification, and other areas of use.

Wastewater Treatment
HTC products are widely used in wastewater treatment, such as heavy metals removal, organic pollutants, and anionic wastewater.Hydrochar offers several advantages in wastewater treatment, including high efficiency, versatility, low cost, and simple operation, thus realizing significant environmental and economic benefits through the "waste-towealth" approach.Due to its high functional group content, porosity, and surface charge, hydrochar can effectively adsorb heavy metals, organic compounds, and anions in wastewater [75].By modifying hydrothermal conditions and adding modifiers, the quantity and types of oxygen-containing functional groups on the surface of hydrochar can be increased, thereby further optimizing the availability of active sites.In Figure 7, HTC's liquid and solid processes for treating wastewater are described.

Application of HTC in Water Environment Management
Water environment management is essential for global sustainable development [29,30,33].HTC technology represents an emerging environmentally friendly approach for revolutionizing global water environmental management.Primarily, this technology leverages the heat and pressure conditions in aqueous environments to transform organic materials into carbon materials with outstanding properties.As a novel green technology, HTC holds broad prospects for application in water environmental governance.This paper will explore in detail the applications of HTC in managing water environments, including wastewater treatment, surface water restoration, groundwater purification, and other areas of use.

Wastewater Treatment
HTC products are widely used in wastewater treatment, such as heavy metals removal, organic pollutants, and anionic wastewater.Hydrochar offers several advantages in wastewater treatment, including high efficiency, versatility, low cost, and simple operation, thus realizing significant environmental and economic benefits through the "wasteto-wealth" approach.Due to its high functional group content, porosity, and surface charge, hydrochar can effectively adsorb heavy metals, organic compounds, and anions in wastewater [75].By modifying hydrothermal conditions and adding modifiers, the quantity and types of oxygen-containing functional groups on the surface of hydrochar can be increased, thereby further optimizing the availability of active sites.In Figure 7, HTCʹs liquid and solid processes for treating wastewater are described.

Use of Hydrochar for Heavy Metal Ions
For heavy metal ions, the adsorption mechanisms of hydrochar primarily include electrostatic attraction, ion exchange, complexation, redox reactions, and coprecipitation as chemical adsorption processes, along with physical adsorption [76][77][78].Redox adsorption can facilitate chemical reactions between various anions and mineral components on the surface of hydrochar and heavy metal ions, resulting in precipitation [77].In addition to chemical adsorption, physical adsorption is also an important mechanism for hydrochar adsorption of heavy metal ions.Physical adsorption is mainly due to the van der Waals force or hydrophobic force of heavy metal ions with the hydrochar surface.When

Use of Hydrochar for Heavy Metal Ions
For heavy metal ions, the adsorption mechanisms of hydrochar primarily include electrostatic attraction, ion exchange, complexation, redox reactions, and coprecipitation as chemical adsorption processes, along with physical adsorption [76][77][78].Redox adsorption can facilitate chemical reactions between various anions and mineral components on the surface of hydrochar and heavy metal ions, resulting in precipitation [77].In addition to chemical adsorption, physical adsorption is also an important mechanism for hydrochar adsorption of heavy metal ions.Physical adsorption is mainly due to the van der Waals force or hydrophobic force of heavy metal ions with the hydrochar surface.When dealing Water 2024, 16, 1749 13 of 28 with heavy metal pollution, appropriate hydrochar materials and optimized preparation conditions can be selected according to the specific application scenarios and the characteristics of the target heavy metal ions to improve the adsorption efficiency.At the same time, the adsorption performance of hydrochar can be further improved by means of surface modification and doping with other functional materials.In Figure 8, the main Ce(III) adsorption mechanism onto hydrochar surface is described.In Table 5, some studies show the effect of hydrochar on the adsorption of heavy metals.
Water 2024, 16, x FOR PEER REVIEW 14 of 29 dealing with heavy metal pollution, appropriate hydrochar materials and optimized preparation conditions can be selected according to the specific application scenarios and the characteristics of the target heavy metal ions to improve the adsorption efficiency.At the same time, the adsorption performance of hydrochar can be further improved by means of surface modification and doping with other functional materials.In Figure 8, the main Ce(III) adsorption mechanism onto hydrochar surface is described.In Table 5, some studies show the effect of hydrochar on the adsorption of heavy metals.Amine-modified hydrochar synthesized from black liquor using hexamethylenediamine (HMDA) at hydrothermal temperatures of 150, 180, 200, and 220 °C, and a duration of 0.5 h exhibited a maximum adsorption capacity of Cr(VI) removal of 741.74 mg/g at 45 °C and pH 2 [80].Hydrochar microspheres produced from glucose at 180 °C for 48 h enables highly selective separation of U(VI).At 333.15 K and pH 4.5, the maximum adsorption capacity for U(VI) was 408.36 mg [81].Similarly, hydrochar microspheres synthesized from sucrose at 190 °C for 11 h, activated with KOH demonstrated a maximum adsorption capacity of 704.2 mg/g for methylene blue.The adsorption process was endothermic [82].Phosphoric acid-modified hydrochar produced from banana peels to at 230 °C over 2 h was evaluated for adsorption capabilities of Pb 2+ [83].The evaluation revealed lead adsorption capacities of 359 mg/g and 193 mg/g for dehydrated and fresh banana peel hydrochar, respectively.Moreover, hydrochar produced from willow twigs via HTC at 300 °C for 30 min used as an adsorbent to remove Cu 2+ and Cd 2+ from aqueous solutions exhibited adsorption capacities of 34 mg/g (0.313 mmol/g) for Cd 2+ and 31 mg/g (0.503 mmol/g) for Cu 2+ [84].Furthermore, hydrochar from pine sawdust (SD) at 260 °C for 2 h, was then activated with H2O2 to enhance the Pb 2+ removal capacity [85].The maximum adsorption capacity of Pb 2+ reached 92.80 mg/g at pH 5.0 and 298 K, which is more than 42 times higher than that of the original hydrochar (2.20 mg/g).Hydrochar from synthesized Pseudomonas aeruginosa shells at 200 °C for 20 min was found to be more effective in adsorbing Pb 2+ and Cd 2+ compared to biochar.The adsorption kinetics followed a pseudo-second-order model, describable by the Langmuir isotherm [86].After reaction at 180-220 °C for 9 h, citric acid catalyzed arecanut husk to produce hydrochar.The results showed that compared with the parent biomass, the mass yield of hydrochar was 58.7%, and the fixed carbon increased from 17% to 39.7%, compared with Zn 2+ , Cr 6+ and Ni 2+ , in the aqueous solution with the initial concentration of 100 mg/L.Hydrochar at a dose of 0.1% shows a maximum adsorption rate of Pb 2+ (79.86 mg/g), and hydrochar can be used to remove Pb 2+ from wastewater as it shows a maximum removal efficiency of 95.08% at 25 mg/L [87].Additionally, sulfide-modified magnetic hydrochar (MHC-S4) was prepared by simultaneously supporting the synthesis of nanoparticles with Fe3O4 and grafting a sulfide-containing group onto a carboniferous water prepared by reacting with pinecone at 200 °C for 5 h.MHC-S4 can effectively adsorb Cd (II)/Pb (II) over a wide pH range and Amine-modified hydrochar synthesized from black liquor using hexamethylenediamine (HMDA) at hydrothermal temperatures of 150, 180, 200, and 220 • C, and a duration of 0.5 h exhibited a maximum adsorption capacity of Cr(VI) removal of 741.74 mg/g at 45 • C and pH 2 [80].Hydrochar microspheres produced from glucose at 180 • C for 48 h enables highly selective separation of U(VI).At 333.15 K and pH 4.5, the maximum adsorption capacity for U(VI) was 408.36 mg [81].Similarly, hydrochar microspheres synthesized from sucrose at 190 • C for 11 h, activated with KOH demonstrated a maximum adsorption capacity of 704.2 mg/g for methylene blue.The adsorption process was endothermic [82].Phosphoric acid-modified hydrochar produced from banana peels to at 230 • C over 2 h was evaluated for adsorption capabilities of Pb 2+ [83].The evaluation revealed lead adsorption capacities of 359 mg/g and 193 mg/g for dehydrated and fresh banana peel hydrochar, respectively.Moreover, hydrochar produced from willow twigs via HTC at 300 • C for 30 min used as an adsorbent to remove Cu 2+ and Cd 2+ from aqueous solutions exhibited adsorption capacities of 34 mg/g (0.313 mmol/g) for Cd 2+ and 31 mg/g (0.503 mmol/g) for Cu 2+ [84].Furthermore, hydrochar from pine sawdust (SD) at 260 • C for 2 h, was then activated with H 2 O 2 to enhance the Pb 2+ removal capacity [85].The maximum adsorption capacity of Pb 2+ reached 92.80 mg/g at pH 5.0 and 298 K, which is more than 42 times higher than that of the original hydrochar (2.20 mg/g).Hydrochar from synthesized Pseudomonas aeruginosa shells at 200 • C for 20 min was found to be more effective in adsorbing Pb 2+ and Cd 2+ compared to biochar.The adsorption kinetics followed a pseudo-second-order model, describable by the Langmuir isotherm [86].After reaction at 180-220 • C for 9 h, citric acid catalyzed arecanut husk to produce hydrochar.The results showed that compared with the parent biomass, the mass yield of hydrochar was 58.7%, and the fixed carbon increased from 17% to 39.7%, compared with Zn 2+ , Cr 6+ and Ni 2+ , in the aqueous solution with the initial concentration of 100 mg/L.Hydrochar at a dose of 0.1% shows a maximum adsorption rate of Pb 2+ (79.86 mg/g), and hydrochar can be used to remove Pb 2+ from wastewater as it shows a maximum removal efficiency of 95.08% at 25 mg/L [87].Additionally, sulfide-modified magnetic hydrochar (MHC-S 4 ) was prepared by simultaneously supporting the synthesis of nanoparticles with Fe 3 O 4 and grafting a sulfide-containing group onto a carboniferous water prepared by reacting with pinecone at 200 • C for 5 h.MHC-S 4 can effectively adsorb Cd (II)/Pb (II) over a wide pH range and achieve rapid absorption equilibrium within 25 min.In addition, MHC-S 4 has excellent adsorption properties in a single system, and the maximum single-layer adsorption capacity of Cd (II) and Pb (II) is 62.49 and 149.33 mg/g, respectively [88].[88]

Use of Hydrochar for Organic Pollutants
Hydrochar serves as an effective adsorbent for organic pollutants such as fuels and antibiotics in wastewater.The adsorption mechanisms of hydrochar involve both physical and chemical interactions, including electrostatic interactions, hydrophobic effects, hydrogen bonding, pore filling, partitioning, and interactions between aromatics and cations [76,77,89].Physical adsorption utilizes the surface characteristics of hydrochar, such as porosity and surface area, to adsorb pollutants on its surface or within its micropores [90].Chemical adsorption involves the formation of chemical bonds between the adsorbent's functional groups, such as hydroxyl, carboxyl, and amino groups, and the adsorbates, facilitating effective removal of heavy metal ions through stable chelation or ionic and covalent bonds.The dominant mechanism depends on the structural and physicochemical properties of the organic pollutants and the biomass charcoal.Modifications and preparations of hydrochar alter its physicochemical properties, enhancing its adsorption performance for organic compounds [91].In Figure 9, the adsorption mechanism of hydrochar for organic pollutants was emphasized by describing the mechanism of TC adsorption of hydrochar, modified-hydrochar, and hydrochar-derived ACs.In Table 6, some studies show the effect of hydrochar on the adsorption of organic pollutants.properties of the organic pollutants and the biomass charcoal.Modifications and preparations of hydrochar alter its physicochemical properties, enhancing its adsorption performance for organic compounds [91].In Figure 9, the adsorption mechanism of hydrochar for organic pollutants was emphasized by describing the mechanism of TC adsorption of hydrochar, modified-hydrochar, and hydrochar-derived ACs.In Table 6, some studies show the effect of hydrochar on the adsorption of organic pollutants.Hydrochar from synthesized municipal sludge at 600 °C for 2 h with Zn/Fe modification demonstrated maximum adsorption capacities of 145.0 mg/g for tetracycline (TC) and 74.2 mg/g for ciprofloxacin (CIP) [93].Moreover, hydrochar from produced bamboo shavings at 200 °C for 3 h, assisted by acid treatment, exhibited maximum adsorption capacities of 90.51 mg/g for Congo Red and 72.93 mg/g for 2-naphthol [94].Tian et al. [91] evaluated the adsorption effects of KOH-modified and unmodified hydrochar produced from montmorillonite and rice husks at 180 °C for 16 h on estrogens in water.They found that a 1% mass ratio of KOH to hydrochar maximized the adsorption efficiency.Li et al. [95] prepared hydrochar from rice straw at temperatures ranging from 160 °C to 200 °C and durations from 40 to 70 min.At 298 K and an initial concentration of 0.5 mg/mL, the maximum adsorption capacities for Congo Red, berberine hydrochloride, 2-naphthol, Zn 2+ , and Cu 2+ were 222.1 mg/g, 174.0 mg/g, 48.7 mg/g, 112.8 mg/g, and 144.9 mg/g, respectively.Furthermore, hydrochar produced from coffee husks at 210 °C for 243 h achieves an adsorption capacity of 34.85 mg/g for methylene blue, predominantly through physical adsorption [96].Zbair et al. [97] synthesized hydrochar from Moroccan nut shells (ANS) at temperatures of 180 °C and 200 °C for 6 h.Additionally, hydrochar produced from fructose at 180 °C for 2 h modified with phloroglucinol showed an adsorption capacity of 274.7 mg/g for tetracycline in water [98].The product demonstrated maximum adsorption capacities of 145.0 mg/g for TC and 74.2 mg/g for CIP. [ Bamboo shavings 200 The maximum adsorption capacities for Congo Red, berberine hydrochloride, 2-naphthol, Zn 2+ , and Cu 2+ were 222.1 mg/g, 174.0 mg/g, 48.7 mg/g, 112.8 mg/g, and 144.9 mg/g, respectively. [ Coffee husks 210 • C, 243 h --25.0 mL of the MB solutions (300 mg/L) The adsorption capacity of methylene blue was 34.85 mg/g, mainly by physical adsorption.The hydrochar generated at 200 • C removed about 92% and 95% of BPA and diuron, respectively, with high adsorption efficiency. [97] Fructose 180 • C, 2 h phloroglucinol

mL of tetracycline solutions
The adsorption capacity of the product to tetracycline in water was 274.7 mg/g.[98] Hydrochar from synthesized municipal sludge at 600 • C for 2 h with Zn/Fe modification demonstrated maximum adsorption capacities of 145.0 mg/g for tetracycline (TC) and 74.2 mg/g for ciprofloxacin (CIP) [93].Moreover, hydrochar from produced bamboo shavings at 200 • C for 3 h, assisted by acid treatment, exhibited maximum adsorption capacities of 90.51 mg/g for Congo Red and 72.93 mg/g for 2-naphthol [94].Tian et al. [91] evaluated the adsorption effects of KOH-modified and unmodified hydrochar produced from montmorillonite and rice husks at 180 • C for 16 h on estrogens in water.They found that a 1% mass ratio of KOH to hydrochar maximized the adsorption efficiency.Li et al. [95] prepared hydrochar from rice straw at temperatures ranging from 160 • C to 200 • C and durations from 40 to 70 min.At 298 K and an initial concentration of 0.5 mg/mL, the maximum adsorption capacities for Congo Red, berberine hydrochloride, 2-naphthol, Zn 2+ , and Cu 2+ were 222.1 mg/g, 174.0 mg/g, 48.7 mg/g, 112.8 mg/g, and 144.9 mg/g, respectively.Furthermore, hydrochar produced from coffee husks at 210 • C for 243 h achieves an adsorption capacity of 34.85 mg/g for methylene blue, predominantly through physical adsorption [96].Zbair et al. [97] synthesized hydrochar from Moroccan nut shells (ANS) at temperatures of 180 • C and 200 • C for 6 h.Additionally, hydrochar produced from fructose at 180 • C for 2 h modified with phloroglucinol showed an adsorption capacity of 274.7 mg/g for tetracycline in water [98].

Use of Hydrochar for Inorganic Anions
For inorganic anions, hydrochar typically undergoes chemical adsorption through chemically formed bonds on the surface [77], exhibiting good adsorption efficiency for anions such as phosphate, arsenate, and fluoride in wastewater.For example, the hydroxyl group on the surface of the hydrochar can form hydrogen bonds with the phosphate ions or undergo coordination interactions to remove the phosphate from the wastewater.Similarly, the carboxyl group on the surface can coordinate with the metal ions in arsenic acid to form a stable complex to effectively remove arsenic acid.For fluoride ions, metal oxides such as aluminum and iron on the surface of hydrochar can exchange ions with fluoride ions to reduce the concentration of fluoride ions in water.In addition, good hydrochar chemical stability and corrosion resistance can be in a wider range of pH to maintain good adsorption performance, which makes the hydrochar become the ideal material for dealing with the wastewater containing inorganic anions.In Table 7, some studies show the effect of hydrochar on the adsorption of inorganic anions.
Anoxygenically digested cattail hydrochar was produced by reacting with pure water, acetic acid assisted, and sodium hydroxide assisted HTC at 200-300 • C for 1 h.The study showed that when the HTC temperature increased from 200 • C to 300 • C, the yield using water, acetic acid and sodium hydroxide decreased from 40.2% to 31.6%, 37.5% to 28.3% and 35.7% to 22.7%, respectively.The adsorption capacities of NH 4 + -N and PO 4 3− -P of these hydrochar are 92.6-122.4mg/g and 1.6-15.8mg/g, respectively, which fit the Freundlich model well [99].Hydrothermal modification reduces the number of acidic functional groups, which is beneficial for anion adsorption.Hydrochar synthesized from chicken feathers at 150-170 • C after 1-3 h has a strong affinity for phosphate, and has a good adsorption efficiency for phosphorus removal under acidic conditions [100].The adsorption isotherms for hydrochar suited the Langmuir model better, with the maximum adsorption capacity (q m ) of hydrochar being 21.70 mg/g at 30 • C. Hydrochar synthesized from microalgae at 348 K for 40 min modified with magnesium exhibited a strong affinity for phosphate, with a maximum adsorption capacity of 89.61 mg/g [25].Additionally, the magnesium-containing hydrochar adsorbed phosphate from water mainly through ion exchange.Magnetic hydrochar modified with Fe 3 O 4 was synthesized from sodium alginate at 210 • C for 5 h [101].The results showed that at a dosage of 2 g/L, the removal rates of arsenic and fluoride were around 85%, with maximum adsorption capacities of 26.06 mg/g and 15.64 mg/g, respectively, in individual systems.As (V) solutions (1-100 mg/L), F solutions (1-50 mg/L) At a dosage of 2 g/L, the maximum adsorption capacity of arsenic and fluoride was 26.06 mg/g and 15.64 mg/g, respectively. [101]

Surface Water Restoration
Surface water restoration is a primary focus in current water environment management [102], with HTC technology playing a significant role, particularly in sediment improvement and water quality enhancement in rivers and lakes.Among them, surface water restoration refers to the treatment and restoration of polluted or damaged surface water bodies through a series of hydrothermal carbonization technical measures and management measures to achieve the purpose of improving water quality and restoring water ecosystem functions.By thermally decomposing and carbonizing sediments and deposits, this technology effectively reduces nutrient release from sediments, thereby controlling eutrophication trends in lakes.Additionally, the hydrochar produced exhibits high adsorptive properties, enabling the selective absorption of harmful substances in the water, significantly enhancing water quality and purifying the water body.Following treatment with HTC technology, rivers and lakes gradually restore their natural ecosystems, providing favorable habitats for diverse aquatic life forms and yielding substantial economic and environmental benefits [103].In Table 8, studies through surface water restoration highlight the role of hydrochar.
Using Prunus serrulata bark as raw material, hydrochar was prepared by a reaction at 200 • C for 6 h, and the product has high efficiency and low precursor material cost, which can remove atrazine from river water.When the acidic pH = 3 is favorable for adsorption, the ideal adsorbent dose is 0.8 g/L, and kinetic studies show that the concentration does not affect the system equilibrium time, which is reached at 240 min.The Langmuir model presented the greatest compliance to the isotherm data and indicated a higher affinity between atrazine and hydrochar, reaching a maximum adsorption capacity of 63.35 mg/g [104].Additionally, extending the reaction time proved to be beneficial for improving the quality of the hydrothermal biochar.Water hyacinth was employed as a feedstock to produce hydrochar at 240 • C and various durations from 0.5 to 24.0 h [105].The results revealed that the specific surface area of the hydrochar was initially increased and then decreased over time.This process not only utilizes the invasive water hyacinth but also achieves ecological restoration.Hydrochar prepared from glucose at 180 • C for 10 h and composited with FeCl 3 •6H 2 O and MnCl 2 •4H 2 O to successfully load MnFe 2 O 4 on its surface could rupture algal cells and disrupt their photosynthetic systems, demonstrating its capability to treat organic matter [106].Water hyacinth was modified with MgCl 2 •6H 2 O and AlCl 3 •6H 2 O, and reacted at 150°C for 24 h to obtain Mg/Al-layered double hydroxides modified water hyacinth hydrochar (MgAl@WH) with layered surface and many adsorption active sites.
The maximum adsorption capacity is 311.0 mg/g, and the adsorption efficiency is about 98.0% after 10 cycles [107].Moreover, research on algal raw materials offers a potential solution for managing eutrophication in lakes and rivers.
Simultaneously, an appropriate amount of hydrochar can enhance the water and soil environment, thereby promoting plant growth and facilitating ecological restoration.Numerous studies have demonstrated that the carbon and magnesium content in hydrochar can influence the uptake of critical nutrients, such as ammonium salts and phosphates [108].Additionally, hydrochar can enhance sulfur levels in the soil and decrease the available nitrogen, and thereby enhance soybean growth [109].However, excessive application of hydrochar may inhibit crop growth, particularly during the seed germination and seedling stages [109][110][111].

Groundwater Purification
Groundwater, as a crucial water resource, is the focus of intense protection and management efforts.Using HTC technology in groundwater purification aims to remove heavy metals and organic pollutants, reduce their bioavailability, and thus inhibit microbial growth and reproduction, ensuring groundwater purity and safety.HTC technology is utilized to prepare hydrochar with diverse adsorption capabilities for various pollutants.Employed as an adsorbent with specific particle sizes, it facilitates efficient underground transport.The impact of carbon-based materials on pollutant removal depends on their physicochemical properties, which are determined by the type of raw material and the pyrolysis conditions used for production.The increase in additives can change the physicochemical properties of carbon-based materials, including biochar and hydrochar, to remove different types of pollutants, which is helpful to develop remediation technologies for contaminated groundwater [112].This enables the removal of organic substances and heavy metal ions from groundwater, safeguarding water source safety and promoting sustainable groundwater use.In Table 8, studies through groundwater purification highlight the role of hydrochar.Hydrochar prepared from switchgrass at 300 • C for 30 min was utilized to remove U (VI) contaminants from groundwater [113].The removal of pathogenic rotavirus (RV) and human adenovirus (HAdV) was investigated by hydrothermal treatment of swine feces (2 h at 180 • C and 7 h at 230 • C) under two conditions, using a 10 cm bed height sand column and with or without an aqueous charcoal supplement (1.5%, w/w).The results showed that the removal efficiency of the two viruses in the improved hydration column was >3 logarithm (complete removal).Although different HTC conditions lead to different characteristics of 180 HTC and 230 HTC, there is no significant difference in virus removal performance between the two hydrates.Hydrochar derived from fecal waste can be used as a capacity grade virus adsorbent [114].Prepared by anaerobic digestion of swine manure at 180 • C for 45 min, hydrochar has been shown to stimulate the growth of denitrifies without toluene degradability, including Candidatus Competibacter and Ferrovibrio, and increase NO 3 − -N removal, which was mainly attributed to the partial denitrification [112].Following ball milling treatment, it exhibited excellent underground transport properties, making it effective for removing TCE from groundwater.Currently, HTC technology is a research hotspot under the "dual carbon" goals.This technology utilizes high-temperature and high-pressure hydrothermal conditions to transform organic matter into carbonaceous materials [21][22][23][24].Compared to traditional carbonization methods, HTC offers advantages such as lower energy consumption, minimal pollution, and enhanced performance of carbon materials.More importantly, HTC can convert carbon in organic wastes into stable carbonaceous materials and steam, significantly reduce carbon emissions from the transformation of organic matter into carbon materials and avoid greenhouse gas emissions.This process effectively sequesters carbon from biomass, which is crucial for achieving the "dual carbon" goals.Additionally, HTC technology can recycle and utilize the energy from organic wastes, further enhancing energy efficiency.

Comparison with Other Environmental Governance Technologies
HTC technology exhibits unique advantages in environmental management.Compared to traditional environmental management technologies, it is more eco-friendly and can significantly reduce secondary pollutants during the treatment of organic wastes, which is fully aligned with current green and low-carbon development concepts [25][26][27].Additionally, this technology facilitates resource utilization by converting organic wastes into high-value products such as activated carbon and carbon black [32].This not only enhances the economic value of wastes but also yields significant social benefits.Notably, HTC processes organic wastes under high temperatures and pressures, achieving high processing efficiency and energy recovery rates, which underscores its effectiveness in environmental management.These advantages underscore the broad potential of HTC technology in environmental management.
Traditional sewage treatment technologies, such as the activated sludge method and biofilm method, can effectively remove organic matter and nutrients in wastewater, but the treatment process may produce secondary pollution, such as odor, noise and chemical residue.The HTC technology can significantly reduce these secondary pollutants, which is fully in line with the current concept of green and low-carbon development.Anaerobic fermentation technology is mainly used to treat high concentration of organic wastewater, Water 2024, 16, 1749 20 of 28 and the biogas produced is a kind of clean energy.However, for some wastewater with low concentrations and a complex composition, the treatment effect of anaerobic fermentation technology may not be ideal.HTC technology can deal with organic waste of various components to achieve high processing efficiency and energy recovery.In Table 9, different water environmental treatment technologies are compared in terms of advantages, Shortcoming and energy consumption.
However, the HTC technique also has some limitations.High-temperature and highpressure conditions of HTC technology may increase equipment costs compared to conventional wastewater treatment technologies, and the procurement and processing of biomass or carbon-containing waste may pose environmental challenges.In order to fully exploit the advantages of HTC technology and mitigate its weaknesses, further research and practice are needed.This includes research on technology optimization, reduced equipment costs, and a sustainable supply of biomass or carbon-containing waste.

Experiments with HTC Technology
Using the Nanjing Qiaobei Sewage Treatment Plant as an example, Wang et al. [115] applied HTC technology to sludge treatment, combined with physicochemical pretreatment, namely Fenton oxidation (FO) and HTC technology to utilize SSL, and found that moderate oxidation could improve the output and performance of the produced hydrochar.The results showed that the pretreatment affected the surface structure and organic composition of SSL and promoted the carbonization of intermediate products.Moreover, compared with the yield of hydrochar (50.7%) obtained via the direct HTC treatment, the yield of the hydrochar obtained using the combined process increased to 55.0-65.2%(depending on the pretreatment condition).Hydrochar properties were enhanced using the combined process, and the energy density of hydrochar slightly decreased after pretreatment (1.2-13.1%);however, the energy yields increased by 0.6-30.1% due to the enhanced hydrochar yield.The carbonization degree of hydrochar was improved; the carbon in the feedstock distributed to the hydrochar increased from 33.40% to 46.09%.The treated sludge not only was effectively reduced but also enabled the recovery and reuse of energy and materials, thereby enhancing the waste's resource utilization.Furthermore, it adsorbs heavy metal ions in wastewater, contributing to water environmental protection.The application of this technology not only reduces carbon emissions but also effectively addresses challenges in sludge treatment, achieving resource recycling.This provides a valuable reference for urban sludge disposal and resource utilization in China.
During the research process, several successful experiments have provided valuable insights.For example, Shi et al. [116] utilized HTC technology to process large particles (>2 cm) of kitchen waste, successfully converting them into carbon materials.This not only addressed wastes disposal issues but also created a new supply source for the carbon materials market.This experiment demonstrates that technological innovation and application can facilitate a win-win situation for economic development and environmental protection.
Experimental studies serve as an effective means to verify the application effects of HTC technology.Scholars have explored the application of HTC technology in various domains, including agricultural wastes, urban trash, and industrial wastewater [117].These experimental studies demonstrate that while HTC technology is effective in practical applications, it also faces several challenges that need to be addressed.Challenges include the pretreatment and post-treatment of biomass or carbon-containing wastes and scaling up and industrializing the equipment.

Technical Challenges
Although the principle of HTC technology is relatively simple, its practical application requires precise control of parameters such as temperature, pressure, and time to achieve optimal carbonization effects [26,118,119].Additionally, the adaptability of this technology to various raw materials poses a technical challenge, as organic wastes of different origins and properties may require tailored processing conditions.Therefore, further optimizing the technological process and enhancing carbonization efficiency are critical challenges that need to be addressed [118,119].Currently, this technology remains in the research and development stage and requires further optimization and enhancements to achieve more efficient and stable production.Furthermore, the widespread adoption and application of this technology must overcome several obstacles, including challenges related to equipment design, manufacturing, operation, and maintenance.

Economic Costs
Economic cost is a significant challenge facing the application of innovative technology [120][121][122].While the long-term economic benefits of HTC technology are substantial, the initial investment is considerable, encompassing equipment purchases, installation, commissioning, and personnel training.Additionally, operational energy consumption and maintenance costs are non-negligible.These factors make it prohibitive for economically disadvantaged areas, potentially limiting its adoption in certain regions.Therefore, it is essential to explore strategies to reduce production costs and enhance economic efficiency, enabling broader adoption of this technology by more regions and enterprises.

Environmental and Social Impact
Environmental and social impact will be closely related to innovative technology application [123,124].When used to process organic wastes, HTC technology can reduce greenhouse gas emissions and enhance resource utilization efficiency.However, it may also generate by-products and pollutants, such as carbonization residues and wastewater.Improper management of these can lead to secondary environmental pollution.Therefore, in promoting this technology, it is crucial to fully assess its environmental and social impacts and establish comprehensive mechanisms for managing by-products and pollutants to ensure real societal benefits and welfare.

Policy and Management
The policy and management play a crucial role in the application of innovative technology [125,126].Governments should establish policies and regulations to encourage and support the research, development, and application of this technology.Additionally, it is essential to establish regulatory mechanisms, define technical standards, and set regulatory requirements to ensure compliance with national and international standards and prevent potential environmental and social risks, thereby providing a robust legal framework for the technology's application.

Summary of the Main Research Results
In the research and application of HTC technology, we have achieved remarkable results.Compared with other wastewater treatment techniques, this technique has been successfully applied to various wastewater treatments, showing effective effects and stability.Moreover, HTC technology has obvious advantages in improving the efficiency of resource recovery extracted from wastewater.It not only transforms organic substances into valuable materials but also can produce high-value carbon materials suitable for different fields, realizing the recycling and reuse of resources.
The application of HTC technology in water environment management provides a new idea and method for water environment management in the world.HTC technology can not only effectively deal with water environment pollution and improve the quality of water environment, but also has obvious advantages in improving the efficiency of resource recovery, reducing the pressure of the resource shortage in China.In addition, the application of HTC technology can also promote the development of the environmental protection industry and promote the realization of a green economy.

Suggestions for Future Research Directions
Although HTC technology has achieved remarkable results in water environment management, there are still some aspects that need further research and improvement.In my perspective, the study of the HTC reaction mechanism is crucial, which helps to improve the reaction efficiency and optimize the process parameters, so as to ensure the product quality.Therefore, it is necessary to organize more in-depth research for a comprehensive understanding and grasp of the HTC reaction mechanism.In addition, the application scope of HTC technology also needs to be further expanded to adapt to various water quality and flow conditions.To this end, the HTC technology is needed to be optimized and improved to make it suitable for more kinds of wastewater treatment, including industrial wastewater, urban sewage, etc.Finally, enhanced integration with other wastewater treatment technologies is the key to developing more efficient and environmentally friendly wastewater treatment systems.This can not only improve processing efficiency, reduce operating costs, but also better cope with complex water quality problems.
To effectively promote and apply HTC technology, decision makers should provide necessary support and guidance.On the one hand, decision makers need to increase their investment in HTC technology research and development, and encourage the cooperation between enterprises, universities and research institutions, to jointly promote the development of HTC technology.This can not only accelerate the process of technology research and development, but also promote technological innovation and upgrading.On the other hand, decision makers need to develop environmental policies to encourage enterprises to adopt environmentally friendly wastewater treatment technologies and improve wastewater treatment standards.This can not only reduce environmental pollution, but also promote sustainable economic development.Finally, the establishment of a set of comprehensive standards for the sewage treatment industry is crucial to standardize its development and promote the wide application of HTC technologies.Through these measures, it can effectively promote the development and application of HTC technology and provide strong technical support for solving the water environment problems in China.
In general, the research and application of HTC technology in organic wastewater treatment and resource recovery provides a new solution for water environment management in China and has broad application prospects and important research value.In the future, we should continue to study HTC technology, optimize its treatment effect, reduce its operating cost, so as to better cope with the challenges of water environment management, promote the development of China's water environment protection cause, and make contributions to the construction of a beautiful China.

Figure 1 .
Figure 1.The distribution of products and phosphate in different phases under different thermal treatments [23].

Figure 1 .
Figure 1.The distribution of products and phosphate in different phases under different thermal treatments [23].

Figure 2 .
Figure 2. The number of papers on hydrochars applied in wastewater treatment.

Figure 3 .
Figure 3. Hop map of keywords for articles on the use of hydrochar using VOSviewer1.6.19 software.

Figure 2 .
Figure 2. The number of papers on hydrochars applied in wastewater treatment.

Figure 2 .
Figure 2. The number of papers on hydrochars applied in wastewater treatment.

Figure 3 .
Figure 3. Hop map of keywords for articles on the use of hydrochar using VOSviewer1.6.19 software.

Figure 3 .
Figure 3. Hop map of keywords for articles on the use of hydrochar using VOSviewer1.6.19 software.

Figure 5 .
Figure 5. History of HTC technology development.

Water 2024 ,
16, x FOR PEER REVIEW 12 of 29 Liquid phase circulating HTC Repeatedly recycle by-products of hydrothermal fluid.

Figure 6 .
Figure 6.The general steps of HTC technology.

Figure 6 .
Figure 6.The general steps of HTC technology.

Figure 7 .
Figure 7. Schematic diagram of the HTC process for treating wastewater-liquid and solid flows.

Figure 7 .
Figure 7. Schematic diagram of the HTC process for treating wastewater-liquid and solid flows.

Table 1 .
Research on HTC technology using catalysts.

Table 3 .
Research on HTC at different temperatures.

Table 4 .
Methods for different types of HTC.

Table 5 .
Research on adsorption of heavy metal ions by hydrochar.

Table 6 .
Research on adsorption of organic pollutants by hydrochar.

Table 6 .
Research on adsorption of organic pollutants by hydrochar.

Table 7 .
Research on adsorption of inorganic anions by hydrochar.

Table 8 .
Research on the ecological protection of hydrochar.

Research Advances in the Context of the "Dual Carbon" Goals
4.1.Influence of HTC Technology on Carbon Emission

Table 9 .
Comparison of different water environment treatment technologies.