Water–Energy–Carbon Nexus: Biochar-Based Catalysts via Waste Valorization for Sustainable Catalysis

Abstract

The water–energy–carbon (WEC) nexus provides a systems framework for minimizing trade-offs among water security, energy reliability, and carbon mitigation. Within this framework, waste-derived biochar catalysts offer a circular pathway that simultaneously valorizes residues, reduces process energy demand, and supports carbon management through stable carbon storage and catalytic co-benefits. This review consolidates recent advances in biochar-based catalysts engineered from agricultural, industrial, municipal, and sludge-derived wastes, highlighting how feedstock selection and thermochemical processing, namely pyrolysis, hydrothermal carbonization (HTC), and torrefaction, as well as activation and post-modification (heteroatom doping and metal/metal-oxide incorporation) govern structure–property–performance relationships. The synthesized catalysts have been widely applied in water and wastewater treatment, including adsorption–advanced oxidation process (AOP) hybrids, Fenton-like systems, peroxydisulfate/persulfate (PS) and peroxymonosulfate (PMS) activation, photocatalysis, and the removal of emerging contaminants. They have also demonstrated strong potential in energy conversion processes such as the hydrogen evolution reaction (HER), oxygen reduction and evolution reactions (ORR/OER), biomass reforming, and carbon dioxide (CO₂) conversion. In addition, these materials contribute to carbon management through sequestration pathways, avoided emissions, and life cycle assessment (LCA)-based sustainability evaluations. Finally, we propose a WEC-aligned design roadmap integrating techno-economic analysis (TEA), LCA, and scale-up considerations to guide next-generation biochar catalysts toward robust performance in real matrices and deployment-ready systems.

1. Introduction

The water–energy–carbon (WEC) nexus is an integrated framework that highlights the interdependent relationships among water resources, energy systems, and carbon emissions [1,2]. In such nexus, water is essential for energy production, including cooling in thermoelectric plants and feedstock processing, while energy is required for water extraction, treatment, and distribution [3,4,5]. Carbon flows that include emissions and sequestration pathways might link these systems further, as energy and water processes are major contributors to greenhouse gas outputs. Focusing on a single component while neglecting the others can create unintended trade-offs. For instance, boosting energy production without considering its effects on water and carbon can exacerbate water scarcity and increase emissions [6]. The WEC nexus thus emphasizes a holistic, systems-level understanding to optimize resource use, improve eco-efficiency, and inform sustainable strategies that simultaneously balance water security, energy reliability, and carbon reduction goals [7,8,9].

This interconnected perspective has gained prominence as policymaking and research evolve beyond isolated sector management toward integrated approaches that enhance resilience and sustainability [10,11]. Frameworks for WEC analysis often combine techno-economic optimization with environmental metrics to reveal key interactions and prioritize interventions that achieve multiple sustainability outcomes, such as minimizing energy-intensive water treatment carbon footprints or leveraging renewable energy to reduce both water use and emissions [12,13,14].

Sustainable catalysis plays a critical role in the transition toward environmentally friendly chemical processes by enhancing reaction efficiency, selectivity, and energy utilization, thereby minimizing waste and reducing reliance on hazardous chemicals. Catalysts lower the activation energy of reactions, enabling processes to proceed under milder conditions and reducing overall energy consumption and greenhouse gas emissions, which aligns with core principles of green chemistry and sustainable industrial practices [15].

The circular economy paradigm supports sustainable catalysis by promoting the recovery and valorization of waste-derived resources. Instead of being discarded, agricultural residues, industrial by-products, and municipal wastes can be converted into functional catalytic materials. Biochar-based catalysts represent a notable example of this strategy, where carbon-rich biomass wastes are transformed into catalytic supports or active materials with significant environmental and economic value [8,16]. Such approaches contribute to waste minimization, resource efficiency, and the closing of material loops, thereby aligning catalytic material development with circular economy principles and broader sustainability objectives [17,18,19,20].

Biochar, a carbon-rich material produced through the thermochemical conversion of biomass under oxygen-limited conditions, has attracted considerable attention as a versatile catalyst or catalyst support for various chemical and environmental applications. Its physicochemical characteristics, including high surface area, tunable porosity, and abundant surface functional groups, facilitate efficient adsorption of reactants, improved electron transfer, and enhanced exposure of catalytically active sites [21,22,23,24]. Furthermore, the catalytic performance of biochar can be tailored through heteroatom doping (e.g., N, S, or P) or incorporation of metal nanoparticles, which expands its applicability in oxidation, reduction, photocatalytic, and electrocatalytic processes [8,11,22,25,26,27]. The structural stability and reusability of biochar further enhance its potential as a sustainable catalytic material by reducing catalyst consumption and minimizing environmental impact [28,29,30].

In addition to conventional catalytic reactions, biochar-based catalysts have demonstrated increasing relevance in environmental remediation and energy-related applications. These materials have been widely investigated for pollutant degradation in water, CO₂ conversion, and catalytic processes involved in bioenergy production [31,32,33,34,35]. By transforming waste biomass into functional catalytic materials, biochar technologies contribute simultaneously to waste management and resource recovery, reinforcing the link between catalytic material development and circular economy practices [36,37,38,39].

Despite these advances, a comprehensive understanding of how biochar-based catalysts contribute to the broader water–energy–carbon (WEC) nexus remains limited. Most existing studies focus on individual applications such as pollutant removal or energy conversion, whereas the integrated role of waste valorization, catalytic functionality, and sustainability within the WEC framework has not been systematically evaluated. Therefore, this review aims to provide a holistic perspective on biochar-based catalysts derived from diverse waste streams, highlighting their physicochemical tuning strategies and multifunctional roles in environmental and energy systems [33,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54]. By emphasizing the intersection of circular economy principles, sustainable catalysis, and WEC nexus challenges, this work proposes a framework for designing next-generation biochar catalysts that are efficient, environmentally benign, and aligned with global sustainability goals [55,56].

2. Bibliometric Analysis

To provide a quantitative overview of research trends in biochar, a bibliometric analysis was conducted using the Web of Science database. The search included the terms “catalysis” AND “biochar” and related keywords, covering publications from 2000 to 2026. Only peer-reviewed articles and reviews in English were considered. This analysis aims to identify research hotspots, influential authors, and emerging directions in the field. The retrieved records were exported and further analyzed using VOSviewer software (version 1.6.19) to visualize bibliometric networks, including keyword co-occurrence, co-authorship patterns, and citation relationships. This approach enables the identification of major research hotspots, influential contributors, and emerging thematic directions in the field.

2.1. Annual Publication Growth in Waste-Derived Biochar

The annual publication trend provides valuable insight into the expanding scientific attention toward waste valorization and biochar-based catalysis. The bibliometric analysis covers the period 2000–2026, since research output on biomass valorization and biochar-based catalytic systems began to increase significantly after 2000, whereas publications prior to this period were relatively scarce and fragmented. The annual publication trajectory from 2000 to 2026 reveals a strong and continuously accelerating expansion of scientific output in the field of waste-derived biochar and its integration within the water–energy–carbon (WEC) nexus. As illustrated in Figure 1, research activity remained relatively modest during the early period (2000–2010), reflecting the foundational stage in which biochar was primarily investigated for soil amendment and carbon sequestration purposes. A noticeable increase emerges after 2012, coinciding with the growing adoption of circular economy principles and the recognition of biochar as a multifunctional platform for environmental remediation and catalytic applications. The most pronounced surge occurs after 2018, with an exponential rise in publication volume throughout 2020–2026. This rapid escalation highlights the increasing global emphasis on sustainable waste valorization strategies, advanced oxidation and adsorption-based water treatment technologies, renewable energy conversion pathways, and carbon management solutions. Hence, the observed publication growth confirms that biochar-based catalysis has evolved into a major interdisciplinary research hotspot aligned with global sustainability and decarbonization agendas.

2.2. Co-Authorship Network Analysis

The co-authorship network visualization generated using VOSviewer illustrates the collaborative structure of research on solid waste–derived biochar. As shown in Figure 2, authors are organized into several distinct clusters, each representing an active research community linked through frequent co-publications. Larger nodes, including Yuan Yong, Ok Yong Sik, Duan Xiaoguang, and Zhou Zhi, correspond to highly productive and influential contributors who act as central hubs within their respective networks. The presence of multiple clusters highlights the interdisciplinary and international character of biochar research, spanning waste valorization, environmental catalysis, pollutant remediation, and carbon management. Strong linkages among core authors indicate well-established collaborations, whereas smaller peripheral nodes reflect emerging researchers and expanding partnerships. Overall, the network suggests that the field is driven by several leading research groups with increasing global connectivity, emphasizing the growing role of collaborative efforts in advancing sustainable biochar-based catalytic technologies within the water–energy–carbon nexus framework.

2.3. Country Collaboration Network Analysis

The country-level collaboration network generated using VOSviewer provides insight into the global research landscape of solid waste–derived biochar and its catalytic applications. As illustrated in Figure 3, the map reveals that the People’s Republic of China serves as the dominant hub, exhibiting the largest node size and the strongest international linkages, reflecting its leading contribution and extensive collaborative output in this field. The United States, Australia, India, and several European countries (e.g., Germany and France) also appear as key contributors, forming interconnected clusters that highlight active cross-regional partnerships. Emerging collaborations are further observed among countries in Asia, the Middle East, and South America, indicating the expanding global interest in biochar-based technologies. Overall, the network demonstrates that research on biochar valorization and sustainable catalysis is increasingly driven by international cooperation, supporting its growing relevance within the water–energy–carbon nexus and worldwide environmental sustainability efforts.

2.4. Keyword Co-Occurrence

Major research themes and emerging hotspots in solid waste–derived biochar studies. As illustrated in the map, “biochar” appears as the dominant central node, strongly connected with key application-oriented terms such as “adsorption,” “removal,” “degradation,” and “oxidation,” highlighting the prominence of environmental remediation as a core research focus. Several thematic clusters can be identified, reflecting the multidisciplinary nature of the field. For instance, one cluster emphasizes pollutant control and water treatment, linking biochar with “aqueous solution,” “organic contaminants,” and “advanced oxidation processes.” Another cluster is associated with energy-related applications, including “biomass conversion,” “gasification,” “hydrogen production,” and “bio-oil,” demonstrating the integration of biochar within waste-to-energy and circular economy strategies. Additional clusters highlight soil remediation and carbon management topics, such as “carbon sequestration,” “phosphate removal,” and “soil amendment.” Overall, the network reveals that biochar research has evolved beyond simple adsorption studies toward broader multifunctional roles spanning catalysis, energy conversion, and sustainability within the water–energy–carbon nexus. The keyword co-occurrence relationships that define the core research domains of solid waste–derived biochar are presented in Figure 4.

3. Waste Valorization for Biochar Production

To realize the full potential of biochar-based catalysts within the water–energy–carbon nexus, it is essential to understand the origin and processing of the feedstocks used for their synthesis [33]. Waste valorization not only provides a sustainable source of carbon-rich materials but also determines the structural, chemical, and catalytic properties of the resulting biochar. The type of biomass, pyrolysis conditions, and post-treatment strategies directly influence surface area, porosity, functional groups, and dopant incorporation, which in turn govern catalytic performance. In the following sections, we review the various waste streams suitable for biochar production, the methods employed to convert them into functional catalysts, and the ways in which these processes can be optimized to support multifunctional applications in environmental remediation and energy conversion [45].

3.1. Types of Feedstocks

The selection of feedstock is a critical determinant of biochar quality, catalytic activity, and functional performance. Biochar is produced via the thermal decomposition of biomass under oxygen-limited conditions, and the nature of the original feedstock strongly influences its physicochemical properties, which include porosity, surface chemistry, elemental composition, and functional groups, which in turn govern its behavior in specific applications such as soil amendment, catalysis, or contaminant adsorption [34,35].

3.1.1. Agricultural Residues

Agricultural residues such as rice husks, corn stover, sugarcane bagasse, wheat straw, and nut shells are abundant, renewable, and widely available, making them sustainable feedstocks for biochar production. These residues are particularly rich in lignocellulosic carbon, composed primarily of cellulose, hemicellulose, and lignin, which largely dictate the structural and chemical properties of the resulting biochars. During pyrolysis, the decomposition of cellulose and lignin contributes to the formation of well-developed pore networks and high specific surface areas, which are critical for adsorption, catalysis, and other surface-dependent applications [36]. The relatively low ash content of these residues compared to manure or industrial by-products results in biochars with moderate pH and lower mineral content, which can be advantageous for applications where soil alkalinity or excessive nutrient loading must be avoided. Moreover, the abundant aromatic structures formed during pyrolysis confer chemical stability and resistance to microbial degradation, making agricultural residue-derived biochars effective for long-term carbon sequestration in soils.

These biochars are highly suitable for a variety of environmental and agronomic applications, including adsorption of organic and inorganic pollutants from water or soil, support for catalytic reactions, and soil amendment to improve porosity, water retention, and microbial habitat. Additionally, the surface functional groups (e.g., hydroxyl, carboxyl, and phenolic groups) generated during pyrolysis enhance their reactivity and ability to interact with nutrients, pollutants, or catalysts [57].

By selecting specific agricultural residues and tailoring pyrolysis conditions, it is possible to customize biochar properties, such as pore size distribution, surface area, and surface chemistry, to meet the requirements of targeted applications in soil remediation, water treatment, carbon capture, and sustainable agriculture, thereby maximizing both environmental and economic benefits.

3.1.2. Industrial By-Products

Industrial by-products that include sawdust, pulp and paper sludge, food processing residues, and agro-industrial husks, which offer significant opportunities for waste valorization by converting materials that would otherwise contribute to landfilling or environmental pollution into value-added biochars [37,38,39,40]. Utilizing these residues not only mitigates waste disposal challenges but also contributes to circular economy principles by recovering nutrients and carbon from otherwise underutilized streams.

The physicochemical properties of biochars derived from industrial by-products are highly dependent on the original feedstock composition, particularly the inorganic mineral content, lignocellulosic structure, and inherent ash content [41,42]. For instance, biochars produced from pulp and paper sludge often have elevated calcium and magnesium concentrations, which can contribute to soil liming effects and pH buffering, while husk-derived biochars may be rich in silica, enhancing soil structure and water retention. Similarly, food processing residues such as fruit pomace or nut shells can retain potassium, phosphorus, and trace micronutrients, which improve fertility and plant nutrient availability [43,44].

These mineral-rich biochars also exhibit enhanced cation exchange capacity (CEC), enabling more effective retention of essential nutrients and reduction in nutrient leaching when applied to soils. In addition, the porosity and surface functional groups generated during pyrolysis can support the adsorption of pollutants, including heavy metals and organic contaminants, thus extending their application to soil remediation and environmental management.

By carefully selecting and processing industrial by-products, it is possible to tailor biochar properties for specific agronomic and environmental applications, such as improving nutrient cycling, enhancing soil structure, or mitigating contamination. This approach not only valorizes waste streams but also contributes to sustainable resource management and the reduction in industrial environmental footprints.

3.1.3. Animal Manures and Organic Waste

Animal manures and organic wastes, such as poultry litter, cow manure, and municipal biosolids, are nutrient-rich feedstocks that serve as excellent precursors for biochar production due to their inherent content of nitrogen (N), phosphorus (P), potassium (K), and micronutrients. Compared to lignocellulosic plant-based feedstocks, manure-derived biochars typically exhibit higher ash content and elevated pH values, which are largely a result of the mineral fraction present in the original biomass. This high mineral content contributes to improved cation exchange capacity (CEC), enabling these biochars to retain and exchange essential nutrients more effectively in soils [45].

The elevated pH of manure-derived biochars can help ameliorate acidic soils, creating a more favorable environment for plant growth and microbial activity. Additionally, the higher ash and nutrient concentrations often translate into greater buffering capacity, reducing soil acidification and enhancing long-term nutrient availability. These characteristics make manure-based biochars particularly suitable for soil remediation, where they can immobilize heavy metals and other contaminants, and for fertility enhancement, where they act as slow-release nutrient reservoirs. Moreover, the porous structure developed during pyrolysis can improve soil water retention and aeration, while the functional groups present on the biochar surface (e.g., carboxyl, hydroxyl, and phosphate groups) can facilitate adsorption of both nutrients and pollutants. Therefore, the selection of animal manure or organic waste feedstocks allows for tailoring biochar properties to specific environmental and agricultural applications, balancing soil health improvement with sustainable waste valorization [46,47].

3.1.4. Municipal Wastes and Urban Organic Fractions

Other municipal organic wastes, including yard trimmings, food scraps, paper waste, and mixed organic fractions of MSW, also represent promising feedstocks for biochar production. Although the heterogeneous composition of these wastes can lead to variability in the physicochemical properties of the resulting biochars, their utilization offers significant environmental benefits. Converting such municipal organics into biochar not only diverts substantial amounts of waste from landfills, thereby reducing methane emissions and leachate generation, but also contributes to circular economy objectives by transforming low-value residues into value-added products. These biochars can serve multiple purposes, including pollutant adsorption, such as removal of heavy metals and organic contaminants from water or soil, and soil amendment, enhancing fertility, water retention, and nutrient cycling. Furthermore, by tailoring pyrolysis conditions to specific feedstock compositions, it is possible to optimize the structural and chemical characteristics of the biochar to suit targeted environmental or agronomic applications [48].

3.1.5. Other Biomass Types

Other biomass types, such as aquatic biomass (e.g., microalgae, macroalgae, and seaweed) and forestry residues (e.g., bark, branches, and logging residues), further expand the range of feedstocks for biochar production. Aquatic biomass is rich in proteins, polysaccharides, and inorganic elements such as nitrogen, phosphorus, and trace metals, which can be retained in the biochar and enhance cation exchange capacity, nutrient availability, and catalytic activity. During pyrolysis, nitrogen- and sulfur-containing compounds in algae can produce heteroatom-doped biochars with active sites suitable for redox reactions, electrocatalysis, and pollutant adsorption. Forestry residues, with their high lignin content, yield thermally stable, aromatic biochars with well-developed microporosity and high surface area. Minerals naturally present in wood, including potassium, calcium, and magnesium, can act as in situ activating agents, improving porosity and introducing basic surface sites that facilitate catalytic reactions.

Biochars derived from these feedstocks exhibit unique surface chemistries and functionalities, broadening their application potential in environmental and energy systems. They can serve as adsorbents for heavy metals and organic pollutants, catalysts or catalyst supports for advanced oxidation, photocatalysis, and electrocatalysis, as well as soil amendments to improve fertility, water retention, and microbial activity. By selecting feedstocks and optimizing pyrolysis conditions, it is possible to tailor the structural and chemical properties of biochars from aquatic and forestry biomass to meet specific requirements in water treatment, energy conversion, and carbon management [49]. Hence, the key feedstock characteristics are lignocellulosic content, ash and mineral fractions, moisture content, and inherent nutrient profiles. Not only govern the resulting biochar’s porosity and surface chemistry but also influence its suitability for targeted applications, from carbon sequestration and soil health improvement to catalysis and contaminant remediation [50].

3.2. Pyrolysis and Carbonization Methods

Biochar is primarily produced through pyrolysis or carbonization, which involve the thermal decomposition of biomass under limited or near-zero oxygen conditions. These processes not only stabilize carbon but also generate biochars with distinct physicochemical properties tailored for various applications [51,52]. The pyrolysis temperature, heating rate, and residence time are critical parameters that influence biochar composition, including carbon content, porosity, aromaticity, surface area, and the abundance of reactive functional groups [53]. Slow pyrolysis, conducted at lower heating rates over extended periods, generally yields higher biochar amounts with greater thermal stability and more developed microporous structures, making them suitable for long-term soil amendment, carbon sequestration, and catalysis. In contrast, fast pyrolysis, conducted at rapid heating rates, favors the production of bio-oil, while generating biochar as a lower-yield co-product that often exhibits higher surface reactivity due to less extensive carbonization [54,55,56,57].

Alternative carbonization methods expand feedstock versatility and allow processing of low-value or high-moisture biomass. Hydrothermal carbonization (HTC) converts wet biomass into hydrochar at moderate temperatures (180–250 °C) under pressurized water, avoiding the need for energy-intensive drying. Hydrochar often retains oxygen- and nitrogen-containing functional groups, which can enhance catalytic activity and pollutant adsorption. Torrefaction, a mild thermal treatment at 200 to 300 °C in inert atmosphere, improves biomass grindability, increases energy density, and produces biochars with partially carbonized structure suitable for combustion or catalytic applications. By carefully selecting carbonization methods and tuning process parameters, biochars can be engineered with targeted properties, including hierarchical porosity, surface functionality, and graphitic domains, which are essential for both environmental remediation and energy conversion applications [44,45,46].

To support the discussion on biochar synthesis routes, Figure 5 summarizes the principal thermochemical pathways used for biochar production, including pyrolysis and related carbonization methods. The figure highlights how operational parameters such as temperature, heating rate, residence time, and oxygen availability strongly influence the physicochemical characteristics of the resulting biochar. These conditions determine the extent of carbonization, pore development, and surface functional groups that govern catalytic performance. In general, slow pyrolysis tends to produce more stable biochars with well-developed microporosity, whereas fast pyrolysis generates more reactive chars with higher surface activity. Alternative processes, such as hydrothermal carbonization, expand feedstock flexibility by enabling the conversion of wet biomass under moderate thermal conditions. Overall, the schematic emphasizes that careful selection of thermochemical pathways is essential for tailoring biochar properties to specific environmental and catalytic applications [43,56].

Alternative thermochemical pathways further broaden the versatility of biomass conversion into carbonaceous materials. Among these, hydrothermal carbonization (HTC) enables the transformation of wet biomass into hydrochar at relatively moderate temperatures (180–250 °C) under autogenous pressure in aqueous environments, thereby eliminating the need for energy-intensive drying steps. The resulting hydrochar typically retains a higher content of oxygenated and nitrogen-containing functional groups, which can enhance surface reactivity, catalytic behavior, and adsorption performance toward various pollutants [44]. Another approach, torrefaction, represents a mild thermal pretreatment conducted at approximately 200–300 °C under inert or oxygen-limited conditions. This process partially decomposes hemicellulose and improves biomass energy density, hydrophobicity, and grindability, producing a partially carbonized solid that can serve as a precursor for catalytic materials or as an upgraded biofuel. Through careful selection of these thermochemical conversion routes and optimization of process parameters, biochars with tailored structural features such as hierarchical porosity, enriched surface functionalities, and enhanced aromaticity can be engineered for targeted environmental remediation and energy-related applications [53,57].

3.3. Activation and Post-Treatment Strategies

While pyrolysis defines the fundamental carbon skeleton of biochar, additional activation and post-treatment strategies are widely applied to unlock its full catalytic potential by tailoring porosity, surface chemistry, and reactive functionality. Activation serves as a critical engineering step to transform raw biochar into a high-performance material with enhanced accessibility of active sites and improved interaction with pollutants or reactants [44].

Physical activation typically conducted using steam, CO₂, or limited air oxidation at elevated temperatures, promotes the development of hierarchical pore structures by opening blocked channels and enlarging micropores into mesopores. This process significantly increases the specific surface area, improves mass transfer, and facilitates diffusion of reactants toward catalytic centers, which is particularly beneficial in water treatment and heterogeneous oxidation systems. While, in contrast, chemical activation provides more aggressive surface modification through reagents such as acids (e.g., H₃PO₄, HNO₃), alkaline agents (e.g., KOH, NaOH), or salts (e.g., ZnCl₂, Na₂CO₃). These treatments introduce abundant surface defects, oxygen-containing functional groups, and redox-active sites, thereby enhancing adsorption affinity, electron-transfer capacity, and catalytic reactivity. Importantly, the selection of activating agent enables fine-tuning of surface acidity/basicity, hydrophilicity, and oxidative potential depending on the targeted application [43,44,56].

Beyond activation, post-treatment functionalization further expands the catalytic versatility of biochar. Heteroatom doping with nitrogen, sulfur, phosphorus, or boron alters the electronic distribution within the carbon matrix, generating electron-rich or electron-deficient sites that promote radical generation, catalytic oxidation, and electrochemical activity. For instance, nitrogen-doped biochars often exhibit superior performance in persulfate activation and oxygen reduction due to enhanced conductivity and surface charge polarization. Moreover, metal and metal-oxide incorporation (e.g., Fe, Co, Ni, Cu, Mn, or noble metals) introduces highly active catalytic centers that enable advanced reactions such as Fenton-like oxidation, photocatalysis, and electrocatalysis. Biochar-supported iron catalysts, for example, can efficiently activate H₂O₂ or persulfate to generate reactive oxygen species (ROS), while Co- or Ni-modified biochars have shown promise in hydrogen evolution (HER), oxygen reduction (ORR), and CO₂ conversion pathways. Therefore, these activation and modification strategies elevate biochar from a simple carbonaceous adsorbent into a multifunctional catalytic platform, capable of driving oxidation–reduction reactions, enhancing pollutant degradation, and supporting sustainable energy conversion processes within the broader water–energy–carbon nexus. Figure 6 presents a comprehensive schematic diagram illustrating the primary pathways for engineering biochar from a basic carbonaceous material into a multifunctional catalytic platform through strategic activation and post-treatment modifications [58].

The diagram displayed in Figure 6 highlights three main biochar modification pathways. Physical activation, using steam, CO₂, or air, promotes the development of hierarchical pore structures and increases surface area. Chemical activation, employing acids, alkalis, or salts, introduces oxygen-containing functional groups and tailors the surface chemistry to enhance reactivity. In addition, post-treatment functionalization through heteroatom doping or metal incorporation creates abundant catalytically active sites. Collectively, these modification strategies transform raw biochar into a high-performance material with improved porosity, surface reactivity, and electron-transfer capability, enabling advanced applications in water treatment and sustainable energy conversion within the water–energy–carbon nexus.

3.4. Determinants of Biochar Properties

The physicochemical and catalytic properties of biochar are not inherent but are engineered through the deliberate selection of biomass feedstock and the precise control of processing conditions. This interplay between source material and synthesis parameters serves as the foundational framework for designing biochar with targeted functionalities [34]. Feedstock composition is a primary variable. Lignin-rich biomass (e.g., hardwood, nut shells) yields biochars with highly aromatic, graphitized structures, offering superior thermal stability. In contrast, biomass high in cellulose and hemicellulose (e.g., straw, grasses) tends to generate more porous biochars with higher specific surface areas, providing a critical scaffold for subsequent activation.

Notably, pyrolysis parameters, particularly temperature, exert definitive control over surface chemistry and structure. Low-temperature pyrolysis (typically 300–500 °C) preserves oxygen-containing functional groups (–OH, –COOH), which enhance hydrophilicity, adsorption affinity, and direct chemical reactivity. High-temperature pyrolysis (600–800 °C and above) promotes graphitization, increases electrical conductivity, removes volatile matter, and creates a more structurally stable carbon matrix suitable for harsh catalytic environments. Furthermore, inherent inorganic constituents (e.g., K, Ca, Mg, Si) within the feedstock can act as natural catalysts during pyrolysis, influencing pore development, thermal degradation pathways, and the resulting surface chemistry. The strategic integration of feedstock selection, pyrolysis tuning, and targeted post-treatments enables precise engineering of biochar’s porosity, surface functionality, electronic properties, and stability. These tailored characteristics directly govern performance in advanced applications, transforming biochar from a passive material into an active platform for contaminant degradation, redox catalysis, electrochemical water-splitting, and CO₂ conversion, thereby fulfilling its multifunctional role within the water-energy-carbon nexus.

Table 1. Key Determinants of Biochar Properties and Their Influence on Catalytic Function.

Determinant	Specific Factor	Influence on Biochar Properties
Feedstock Composition	High Lignin Content (e.g., hardwood)	• Promotes highly aromatic, graphitized carbon structure • High thermal stability
	High Cellulose/Hemicellulose (e.g., straw, grass)	• Favors the development of porous structures • Higher specific surface area (SSA)
	Inherent Inorganics (K, Ca, Mg, Si)	• Acts as a natural template/catalyst during pyrolysis • Influences pore development and surface chemistry
Pyrolysis Conditions	Low Temperature (300–500 °C)	• Retains oxygen-containing groups (–OH, –COOH) • Moderate SSA, less graphitized
	High Temperature (600–800+ °C)	• High graphitization and electrical conductivity • Removes volatiles, increases aromaticity and stability
Integrated Engineering	Combination of Feedstock + Pyrolysis + Post-Treatment	• Precise tuning of Porosity (micro/meso), Specific Surface Area, Surface functionality, Electronic structure, Stability

outlines the primary engineering pathways used to transform raw biochar into a multifunctional catalytic material through targeted activation and functionalization strategies [12,33,41].

Furthermore, it is noteworthy to mention that Biochar-based catalysts offer several advantages compared with traditional noble metal catalysts in electrochemical energy conversion processes. One of the primary advantages is their low cost and wide availability, since biochar can be produced from abundant waste biomass such as agricultural residues, industrial by-products, and municipal organic wastes. This waste-derived origin significantly reduces material costs while simultaneously promoting sustainable waste management practices. Additionally, another important benefit is the high surface area and hierarchical porous structure of biochar materials, which enhances mass transport and provides many accessible catalytic active sites. Additionally, the surface chemistry of biochar can be tuned through heteroatom doping, such as N, S, and P, which modifies the electronic structure of the carbon framework and improves catalytic activity. Biochar can also effectively support transition metals such as Fe, Co, and Ni, where strong metal–support interactions enhance the dispersion, stability, and durability of catalytically active centers. Furthermore, biochar-based catalysts contribute to environmental sustainability and circular economy strategies. By converting waste biomass into functional catalytic materials, biochar production supports resource recovery, reduces waste disposal, and enables long-term carbon storage. These characteristics make biochar-based catalysts promising sustainable alternatives to conventional noble metal catalysts in energy-related catalytic applications [57,58,59].

3.5. Structural and Physicochemical Properties of Biochar Catalysts

The catalytic efficacy of biochar-based materials is governed by a well-defined set of structural and physicochemical properties. Key characteristics include specific surface area, hierarchical porosity, morphology, surface functional group chemistry, and elemental composition. A high surface area, coupled with an interconnected network of micro- and mesopores, is fundamental as it facilitates the adsorption of reactants and ensures efficient mass transport to active sites [52]. Concurrently, the surface chemistry, defined by functional groups such as hydroxyl, carboxyl, carbonyl, and phosphate, dictates chemical reactivity, hydrophilicity, and the mechanisms of catalytic reactions, including electron transfer and radical generation [35,36]. Further functionalization through heteroatom doping (e.g., N, S, P) or metal/metal-oxide incorporation strategically modifies the electronic structure of the carbon matrix. This enhances electron density, introduces redox-active sites, and increases the availability of catalytic centers, which are critical for advanced applications like persulfate activation, electrocatalysis, and photocatalytic energy conversion.

A robust understanding of these properties necessitates the application of advanced characterization techniques. Nitrogen physisorption (BET) analysis quantifies surface area and pore size distribution, while electron microscopy (SEM/TEM) reveals morphology and microstructure. Surface chemistry and elemental states are probed by X-ray photoelectron spectroscopy (XPS), and the degree of graphitization and crystallinity is assessed via Raman spectroscopy and X-ray diffraction (XRD). Fourier-transform infrared (FTIR) spectroscopy provides essential information on functional groups. This multi-technique characterization suite enables the rational design and precise tuning of biochar-based catalysts for targeted performance in environmental remediation and sustainable energy applications [26,43].

Systematic physicochemical characterization is essential for linking the structural and surface features of engineered materials to their functional performance in environmental remediation and energy-related applications, as displayed in Figure 7. The schematic illustrates how key analytical techniques, including BET surface analysis, spectroscopy (XPS/FTIR), electron microscopy (SEM/TEM), and structural probes (XRD/Raman), quantify surface area, surface chemistry, morphology, and crystallinity. These engineered properties collectively enable enhanced adsorption and diffusion, improved chemical reactivity, accelerated electron transfer, and increased structural stability, ultimately leading to integrated performance in environmental and energy applications.

3.6. Biochar Applications

3.6.1. Catalytic Applications in Water Treatment

Biochar-based catalysts have emerged as highly promising materials in water and wastewater treatment, owing to their multifunctional ability to serve simultaneously as efficient adsorbents and catalytic mediators. Their intrinsically porous structure, high specific surface area, abundant surface functional groups, and tunable physicochemical properties enable strong interactions with a wide range of aqueous contaminants. Consequently, biochar-derived catalytic systems have demonstrated remarkable effectiveness in the removal and degradation of persistent organic pollutants, including synthetic dyes, pharmaceuticals, phenolic compounds, pesticides, antibiotics, and endocrine-disrupting chemicals, which are often resistant to conventional treatment technologies [27,41].

The integration of biochar-based catalysts into advanced oxidation processes (AOPs) has further expanded their applicability and significantly enhanced treatment performance. Biochar can effectively activate oxidants such as hydrogen peroxide, persulfate, and peroxymonosulfate in Fenton-like and sulfate-radical-driven systems, promoting the generation of highly reactive oxygen species (•OH, SO₄•⁻, and •O₂⁻) capable of rapid contaminant mineralization. Metal-loaded biochar catalysts (e.g., Fe-, Co-, and Mn-based composites) exhibit superior catalytic activity, broader operational pH windows, and reduced metal leaching compared with traditional homogeneous catalysts. Moreover, in photocatalytic systems, biochar functions as an electron mediator, suppressing charge-carrier recombination and extending light absorption, thereby accelerating degradation kinetics under solar or visible-light irradiation [50,59].

Hybrid treatment strategies that couple adsorption with catalytic oxidation provide additional advantages through synergistic mechanisms. Biochar’s strong affinity for organic molecules enables pollutant pre-concentration on catalytically active sites, facilitating localized and rapid degradation. This dual functionality not only improves reaction efficiency and kinetics but also reduces the formation of undesirable secondary by-products. Furthermore, the regeneration and reuse of biochar-based catalysts through thermal, chemical, or electrochemical approaches enhances their long-term economic viability and environmental sustainability. Beyond organic contaminant removal, engineered biochar catalysts have also shown strong potential for the sequestration and transformation of inorganic pollutants, particularly toxic heavy metals such as Hg²⁺, Cu²⁺, Pb²⁺, Cd²⁺, and Cr(VI). Their high cation exchange capacity, oxygen-containing functional groups, and redox-active sites support adsorption, surface complexation, and, in some cases, reduction of metals into less toxic or more stable forms. Increasingly, advanced biochar systems are also being explored for emerging contaminants, including microplastics, per- and polyfluoroalkyl substances (PFAS), and nutrient species, highlighting their versatility in addressing complex water pollution challenges [58,59]. Hence, biochar-based catalytic materials represent cost-effective, scalable, and environmentally benign solutions for next-generation water treatment technologies. By valorizing waste-derived carbon resources and coupling them with catalytic remediation pathways, these systems align strongly with circular economy principles while contributing to sustainable water quality management within the broader water–energy–carbon nexus [44,60,61,62]. Future research should prioritize improving catalyst stability under realistic water matrices, minimizing secondary pollution risks, and developing integrated treatment platforms suitable for large-scale and decentralized applications. Figure 8 illustrates the dual-function mechanism of biochar-based catalysts in water treatment, combining adsorption of diverse pollutants with subsequent radical-mediated oxidation through advanced oxidation processes (AOPs) for comprehensive contaminant removal [58,63].

Table 2. Literature summary of biochar-based catalysts in catalytic wastewater treatment.

Biochar-Based Catalyst System	Biochar Feedstock/Support	Target Pollutant(s)	Treatment Process/Oxidant	Dominant Reactive Species	Key Outcomes/Advantages	Ref.
Fe-loaded biochar (Fe–BC)	Rice husk biochar	Pharmaceuticals (diclofenac, ibuprofen)	Fenton-like (H2O2 activation)	•OH	High degradation efficiency; reduced Fe leaching vs. homogeneous Fenton	[58]
MnOx/biochar composite	Bamboo biochar	Synthetic dyes (MB, RhB)	PMS activation	SO4•−, •OH	Strong oxidation performance under mild pH; good stability	[59]
Co–biochar catalyst	Corn straw biochar	Antibiotics (tetracycline)	Persulfate activation	SO4•−, •O2−	Rapid degradation kinetics; reusable over multiple cycles	[60]
Biochar-supported nZVI (BC–nZVI)	Wood-derived biochar	Heavy metals (Cr(VI), Pb2+)	Adsorption + redox reduction	e− transfer	Synergistic metal sequestration and detoxification	[61]
N-doped biochar (metal-free)	Sewage sludge biochar	Phenols, organic micropollutants	PMS activation	1O2, •O2−	Metal-free catalysis; minimal secondary contamination	[62]
Biochar/TiO2 composite	Agricultural waste biochar	Dyes, endocrine disruptors	Photocatalysis (visible light)	•OH, h+	Enhanced electron transfer; suppressed recombination	[63]
Magnetic biochar (Fe3O4–BC)	Coconut shell biochar	Pesticides (atrazine)	Photo-Fenton-like	•OH	Easy recovery via magnetic separation; high catalytic activity	[64]
Cu–biochar catalyst	Sawdust biochar	Emerging contaminants (PFAS precursors)	PMS oxidation	SO4•−	Promising PFAS degradation; stability still under evaluation	[65]
Biochar catalysts in constructed wetlands	Mixed lignocellulosic biochar media	Mixed wastewater organics	Catalytic CW + ROS generation	ROS + adsorption	Improved removal efficiency through adsorption–oxidation synergy	[66]

provides a concise literature overview of recent biochar-based catalytic systems applied in water and wastewater treatment. The studies collectively demonstrate that biochar is not only an effective adsorbent but also a versatile catalytic platform capable of activating multiple oxidants such as hydrogen peroxide, persulfate, and peroxymonosulfate. Moreover, Metal-supported biochars (e.g., Fe–BC, MnOx/BC, Co–BC, Cu–BC) show enhanced degradation efficiencies through the generation of highly reactive oxygen species, particularly hydroxyl radicals (•OH) and sulfate radicals (SO₄•⁻). These systems often outperform conventional homogeneous Fenton processes due to improved catalyst stability and reduced metal leaching [64]. In parallel, metal-free engineered biochars (e.g., N-doped biochar) offer promising environmentally benign alternatives, minimizing secondary contamination while enabling selective non-radical pathways such as singlet oxygen (¹O₂). Furthermore, hybrid composites integrating photocatalysts (TiO₂) or magnetic phases (Fe₃O₄) highlight additional functional advantages, including improved electron transfer, suppressed recombination, and easy catalyst recovery [65,66]. Importantly, emerging applications such as catalytic biochar integration into constructed wetlands indicate a growing trend toward synergistic adsorption/oxidation treatment under realistic wastewater conditions. Hence, Table 2 emphasizes the multifunctionality, tunability, and sustainability potential of biochar-based catalysts as next-generation materials for advanced water remediation [36,65].

To provide concrete evidence supporting the physicochemical characteristics discussed above, several representative biochar-based catalytic systems reported in the literature are summarized in Table 2. For example, Fe₃O₄-loaded biochar derived from rice husk exhibited a BET surface area of approximately 312 m² g⁻¹, with SEM images revealing a highly porous carbon matrix decorated with uniformly dispersed magnetite nanoparticles. XRD analysis confirmed the crystalline Fe₃O₄ phase with characteristic diffraction peaks at 30.1°, 35.5°, 43.1°, and 57.0°, corresponding to the (220), (311), (400), and (511) planes, respectively. This catalyst demonstrated high heterogeneous Fenton activity, achieving 95% degradation of methylene blue within 60 min under optimized conditions [58,64]. Similarly, iron-modified corn-straw biochar with a surface area of 280 m² g⁻¹ exhibited efficient catalytic oxidation of Rhodamine B, reaching over 90% removal within 50 min due to the synergistic effects of adsorption and Fe-mediated radical generation [58,60]. Raman spectroscopy of such biochar systems typically shows the characteristic D band (1350 cm⁻¹) and G band (1580 cm⁻¹), reflecting disordered and graphitic carbon domains that facilitate electron transfer during catalytic reactions [59,62]. In another example, nitrogen-doped sewage-sludge biochar with surface areas exceeding 400 m² g⁻¹ has been reported to enhance catalytic oxidation performance through the introduction of surface defect sites and heteroatom functionalities that improve electron mobility and reactive oxygen species generation [62,65]. These representative systems clearly demonstrate how structural characteristics, including surface area, pore structure, heteroatom doping, and metal nanoparticle dispersion, govern the catalytic behavior of biochar-based materials in advanced oxidation processes [58,59,60,61,62,63,64,65,66]. Incorporating such quantitative examples helps clarify the structure–activity relationships underlying the performance of biochar-derived catalysts.

3.6.2. Catalytic Applications in Energy Conversion

Biochar-based catalysts have emerged as versatile and sustainable materials for diverse energy conversion applications, leveraging their hierarchical porosity, tunable surface chemistry, and favorable electronic properties. In hydrogen-related technologies, biochar-supported catalysts show significant promise for hydrogen production through water electrolysis, biomass reforming, and photocatalytic processes. Heteroatom doping with elements such as nitrogen, sulfur, phosphorus, or boron that enhances the electrical conductivity of biochar frameworks and optimizes active sites, leading to improved performance in the HER. In electrochemical energy systems, biochar-derived carbons are increasingly studied as cost-effective alternatives to noble-metal catalysts in fuel cells and metal–air batteries. Their high surface area and defect-rich structures facilitate efficient oxygen adsorption and activation, enabling effective catalysis of both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Metal-free heteroatom-doped biochars, as well as biochar-supported transition metal catalysts (e.g., Fe, Co, Ni), have demonstrated competitive activity and superior durability compared to conventional carbon black supports, particularly under alkaline conditions [67,68].

Biochar-based catalysts also contribute to CO₂ conversion and utilization, supporting carbon management strategies. Biochar-supported metal nanoparticles promote reactions such as CO₂ hydrogenation, dry reforming of methane, and electrochemical CO₂ reduction, transforming greenhouse gases into value-added fuels and chemicals. The intrinsic alkalinity and surface functional groups of biochar enhance CO₂ adsorption, while strong metal–support interactions improve catalyst dispersion, sintering resistance, and stability [33,69]. Furthermore, biochar serves as an effective catalyst or catalyst support in syngas production and biomass-to-fuel processes, including gasification, pyrolysis upgrading, and Fischer–Tropsch synthesis. Integrating waste-derived biochar into these systems enables simultaneous waste valorization, renewable fuel production, and reduced reliance on fossil carbon. By combining inherent catalytic activity with carbon sequestration potential, biochar-based catalysts offer a unique pathway toward low-carbon and circular energy technologies [70].

Despite these advances, challenges remain in achieving consistent catalytic performance, long-term stability, and scalable production. Future research should focus on precise control of biochar structure, rational doping and metal loading strategies, and in-depth mechanistic studies of catalytic pathways. Overall, biochar-based catalysts represent a promising and sustainable platform for energy conversion, effectively bridging waste valorization with advanced catalytic applications [68].

To illustrate the multifunctional role of biochar-derived catalysts in sustainable energy technologies, Figure 8 presents a schematic summary of the major catalytic pathways where engineered biochar materials contribute to energy conversion and carbon management. As depicted in Figure 8, biochar-based catalysts represent a versatile platform for a wide range of energy conversion processes due to their hierarchical porous structure, tunable surface chemistry, and ability to host both heteroatom dopants and transition metal active sites [27,28,69]. The schematic highlights four key application domains, including hydrogen production, electrochemical energy systems, CO₂ conversion, and biomass-to-fuel upgrading. In hydrogen-related technologies, doped biochar frameworks enhance electrical conductivity and provide abundant catalytic sites for the HER. In fuel cells and metal–air batteries, defect-rich biochar carbons facilitate oxygen adsorption and activation, enabling efficient catalysis of oxygen reduction and evolution reactions. The figure also emphasizes the growing importance of biochar-supported catalysts in CO₂ utilization pathways, where surface alkalinity and strong metals that support interactions improve adsorption, conversion efficiency, and stability. Additionally, biochar serves as an effective catalyst or support in biomass conversion routes such as gasification, pyrolysis upgrading, and syngas-to-fuel synthesis, supporting renewable fuel production while promoting waste valorization. In this regard, Figure 8 underscores the unique potential of biochar-based catalysts to bridge circular carbon strategies with advanced catalytic energy technologies, offering sustainable alternatives for low-carbon energy conversion systems. Figure 9.

As shown in

Table 3. Representative Literature on Biochar-Based Catalysts in Energy Conversion Applications.

Energy Application	Biochar Catalyst Type	Modification	Target Process	Performance Highlights	Ref.
Hydrogen Production	Biochar-supported electrocatalyst	N-, S-, P-doped biochar frameworks	HER	Heteroatom doping improves conductivity and increases active catalytic sites for HER	[57]
Fuel Cells	Metal-loaded biochar catalyst	Ni, Co, Fe nanoparticles on biochar	Biomass reforming/Electrolysis	Enhanced hydrogen yield due to strong metal–support interactions and high dispersion	[67]
	Metal-free doped biochar	N-doped porous biochar	ORR	Competitive ORR activity vs. Pt-based catalysts, especially under alkaline conditions	[68]
	Transition metal biochar catalyst	Fe–N–C biochar composites	ORR catalysis in PEM fuel cells	High durability and improved oxygen adsorption due to a defect-rich carbon matrix	[69]
Metal–Air Batteries	Biochar-derived carbon catalyst	Co, Ni supported biochar	ORR/OER bifunctional catalysis	Defect-rich biochar improves oxygen kinetics and cycling stability in Zn–air batteries	[70]
CO2 Conversion	Biochar-supported metal catalyst	Cu, Fe, Ni nanoparticles	CO2 hydrogenation/Dry reforming	Enhanced CO2 adsorption and conversion due to alkaline surface groups	[71]
	Electrochemical biochar catalyst	N-doped biochar electrodes	Electrochemical CO2 reduction (CO2RR)	Improved selectivity toward CO/formate with stable long-term performance	[72]
Syngas Production	Waste-derived biochar catalyst	Alkali-rich biochar ash content	Biomass gasification	Promotes tar cracking and syngas yield enhancement	[73]
Pyrolysis Upgrading	Catalytic biochar support	Metal-loaded biochar (Ni/BC, Fe/BC)	Bio-oil upgrading	Reduces oxygenated compounds and improves fuel quality	[74]
Circular Carbon and Waste Valorization	Waste biomass-derived biochar catalysts	Multi-functional catalytic biochars	Integrated biomass-to-fuel pathways	Enables renewable fuel production while supporting carbon sequestration	[75]
Fischer–Tropsch Synthesis	Biochar-supported catalyst	Co/Fe catalysts on activated biochar	Syngas to liquid fuels	High stability and resistance to sintering compared to conventional supports	[76]

, biochar-based catalysts have been widely explored as sustainable alternatives to conventional noble-metal catalysts in multiple energy conversion applications. In particular, heteroatom-doped biochars demonstrate strong potential as metal-free electrocatalysts for oxygen reduction and evolution reactions, benefiting from enhanced conductivity, abundant defect sites, and improved oxygen adsorption behavior. Meanwhile, biochar-supported transition metals such as Fe, Co, and Ni exhibit superior dispersion and stability, making them highly effective in hydrogen evolution, syngas production, and biomass reforming processes. Table 3 also illustrates that biochar materials play an increasingly important role in CO₂ conversion and utilization, where their intrinsic alkalinity and surface functional groups enhance carbon capture while promoting downstream catalytic transformation into value-added fuels and chemicals. Furthermore, waste-derived biochar integration into pyrolysis upgrading and Fischer–Tropsch synthesis supports circular economy strategies by simultaneously enabling renewable fuel generation and waste valorization. Therefore, the studies tabulated in Table 3 confirm that biochar-based catalysts offer a multifunctional platform bridging carbon sequestration, sustainable catalysis, and clean energy production, although challenges remain in achieving scalable synthesis, consistent performance, and mechanistic clarity for industrial deployment.

3.7. Carbon Management and Environmental Impact

Biochar-based materials play a pivotal role in carbon management by stabilizing biomass-derived carbon in chemically recalcitrant forms that can persist in soils for decades to centuries. During thermochemical conversion, a fraction of the original biomass carbon is transformed into aromatic and condensed structures that resist microbial degradation, thereby providing a long-term carbon sink. These characteristics position biochar as a promising negative-emission technology, particularly when derived from waste biomass that would otherwise decompose or be landfilled, releasing greenhouse gases (GHGs).

Life cycle assessment (LCA) studies increasingly demonstrate that biochar production and application can result in net reductions in GHG emissions, depending on feedstock type, conversion technology, and end-use scenario. Emission savings arise from multiple pathways, including avoided methane emissions from landfills, displacement of fossil-based materials or catalysts, and co-generation of syngas or bio-oil for energy recovery during pyrolysis or gasification. When integrated into energy systems, biochar production can enhance overall process efficiency and contribute to low-carbon energy transitions. From an environmental perspective, the valorization of agricultural residues, industrial by-products, and municipal organic wastes into biochar aligns strongly with circular economy principles by closing material loops, reducing waste volumes, and recovering value-added products. This approach not only alleviates pressure on landfills but also minimizes the environmental risks associated with improper waste disposal. Moreover, biochar application in soils has been shown to improve soil fertility, structure, and water-holding capacity, which is particularly beneficial in arid and semi-arid regions where water scarcity is a major concern [77,78,79].

In addition to its role in carbon sequestration, biochar contributes to pollution mitigation and ecosystem protection. Its high surface area and functionalized surfaces enable the immobilization of nutrients, heavy metals, and organic contaminants, thereby reducing leaching losses and improving nutrient use efficiency [54]. These properties can indirectly lower emissions of nitrous oxide (N₂O) from soils and mitigate groundwater contamination, further enhancing environmental sustainability. Despite these benefits, the environmental impacts of biochar are highly context-dependent, and potential trade-offs must be carefully evaluated [80]. Factors such as feedstock contamination, energy intensity of production, and long-term stability under varying environmental conditions require systematic assessment. Future studies should integrate field-scale experiments, long-term monitoring, and standardized LCA frameworks to better quantify the climate and environmental benefits of biochar-based systems. Thus, biochar represents a multifunctional platform that bridges carbon management, waste valorization, and environmental remediation within the broader water–energy–carbon nexus [80,81,82,83,84].

Table 4. Representative literature on carbon management and the environmental impacts of biochar-based materials derived from waste biomass.

Feedstock	Conversion Method	Application	Outcomes	LCA	Ref.
Agricultural residues (rice husk, corn stover)	Slow pyrolysis	Soil amendment	Long-term carbon sequestration; improved soil water retention	Net negative GHG emissions due to stable carbon storage and reduced fertilizer demand	[77]
Forestry waste	Fast pyrolysis	Bioenergy + biochar co-production	Carbon sequestration with energy recovery	GHG reduction depends on energy substitution efficiency	[78]
Municipal solid waste (organic fraction)	Pyrolysis	Waste management and soil application	Avoided landfill methane emissions; stabilized carbon	Up to 40–60% reduction in lifecycle GHG emissions	[79]
Municipal solid waste	Gasification	Syngas production + biochar	Reduced landfill volume; carbon retention in char	Integrated energy recovery improves overall carbon balance	[80]
Sewage sludge	Pyrolysis	Soil remediation	Immobilization of heavy metals; carbon stabilization	Environmental benefit depends on contaminant control	[81]
Agricultural waste	Pyrolysis + activation	Water treatment catalyst	Indirect carbon benefit via pollution mitigation	Emission reductions via extended catalyst lifetime	[82]
Mixed biomass waste	Pyrolysis	Carbon sequestration strategy	Stable aromatic carbon structures	Carbon stability up to centuries under soil conditions	[83]
Crop residues	Pyrolysis	Climate mitigation	2.5–6.6 Gt CO2-eq yr−1 sequestration potential	Strong climate mitigation potential at the global scale	[84]

summarizes representative studies addressing the role of biochar-based materials derived from waste biomass in carbon management and environmental sustainability.

3.8. Water–Energy–Carbon Nexus Framework

The water–energy–carbon (WEC) nexus represents an integrated sustainability framework that captures the intrinsic interdependence among water resources, energy systems, and carbon emissions. Rather than addressing water treatment, energy production, and climate mitigation as isolated challenges, the nexus approach emphasizes their coupled interactions and potential trade-offs. For instance, energy-intensive water purification technologies may inadvertently increase greenhouse gas emissions, while decarbonized energy pathways often require substantial water inputs. Consequently, the WEC nexus promotes a systems-level perspective aimed at optimizing resource efficiency, reducing environmental burdens, and simultaneously advancing water security, energy reliability, and carbon mitigation goals. Within this framework, waste-derived biochar-based catalysts have emerged as a highly promising platform for enabling WEC-aligned solutions [66]. Biochar production valorizes agricultural, industrial, and municipal residues into functional carbon materials, diverting waste from landfills and reducing methane-related emissions. At the same time, engineered biochar catalysts contribute directly to water sustainability through multifunctional roles in pollutant adsorption and catalytic oxidation, enabling efficient degradation of persistent contaminants under relatively mild conditions. These synergistic adsorption–catalysis mechanisms can lower chemical requirements, enhance treatment kinetics, and reduce the overall energy footprint of wastewater remediation, reinforcing the strong water–energy coupling within the nexus [16,22,80].

From an energy conversion perspective, biochar-based catalysts are increasingly explored as sustainable alternatives to conventional noble-metal systems in renewable energy technologies. Their hierarchical porosity, tunable surface chemistry, and capacity to host heteroatom dopants or transition-metal active sites provide favorable electronic and catalytic properties. As a result, biochar catalysts have demonstrated growing relevance in hydrogen evolution, oxygen reduction and evolution reactions in fuel cells and metal–air batteries, biomass reforming, and CO₂ conversion pathways. Importantly, these applications connect waste valorization with clean energy generation, highlighting the potential of biochar catalysts to bridge circular economy principles with low-carbon energy transitions [25,81,84].

Carbon management constitutes the third critical pillar of the WEC nexus, and biochar offers unique advantages through both direct and indirect mechanisms. Thermochemical conversion stabilizes biomass carbon into aromatic, recalcitrant structures that can persist for decades to centuries, positioning biochar as an effective long-term carbon sequestration agent. In parallel, the catalytic deployment of biochar can deliver avoided emissions by displacing fossil-derived catalysts, reducing energy requirements in water treatment, and enabling renewable fuel production from biomass and CO₂ utilization. Collectively, these co-benefits underscore the multifunctional role of biochar catalysts in linking environmental remediation, energy transformation, and climate mitigation within a unified sustainability strategy [77].

To translate the WEC nexus concept into practical catalyst design, consistent performance accounting frameworks are essential. Such frameworks should integrate water treatment efficiency, energy intensity, and net carbon balance within clearly defined system boundaries. Key indicators include pollutant mineralization performance in real matrices, energy demand associated with biochar synthesis and regeneration, catalyst durability, and greenhouse gas impacts evaluated through life cycle assessment. This nexus-based evaluation enables the identification of trade-offs, such as catalytic enhancements achieved through intensive chemical activation versus increased embodied emissions, thereby supporting rational development of scalable and environmentally benign biochar systems [37,66]

A particularly useful operational tool is the nexus decision matrix, which links application constraints to catalyst engineering strategies. In complex water matrices containing high natural organic matter, dissolved salts, or radical scavengers, robust and selective systems such as heteroatom-doped metal-free biochars or encapsulated metal catalysts are often preferred, alongside non-radical activation pathways (e.g., singlet oxygen generation or surface-mediated electron transfer). Where energy availability is limiting, low-temperature hydrothermal carbonization routes, mild activation methods, and solar-assisted photocatalytic hybrids become attractive to minimize synthesis and operational energy penalties. Furthermore, when carbon mitigation is a primary objective, catalyst design should prioritize high fixed-carbon yields, long-term stability, and low embodied emissions, avoiding excessive chemical modification unless performance gains justify the associated carbon costs [12,45]. Thus, the WEC nexus framework highlights the strategic importance of waste-valorized biochar catalysts as multifunctional materials capable of addressing interconnected sustainability challenges. By simultaneously supporting advanced water purification, renewable energy conversion, and carbon sequestration, biochar-based catalytic systems offer a compelling pathway toward low-carbon and circular catalytic technologies. Future research should emphasize mechanistic understanding, standardized reporting metrics, and integrated techno-economic and life cycle evaluations to accelerate the transition of these systems from laboratory demonstrations to deployment-ready solutions within the broader WEC nexus.

3.9. Nexus Design Rules of Water–Energy–Carbon Framework

To guide the rational development of biochar-based catalytic systems that deliver simultaneous benefits across water sustainability, energy efficiency, and carbon mitigation, the following WEC nexus-aligned design rules are proposed. Figure 10 provides a graphical summary of the key nexus design rules that guide the engineering of waste-derived biochar catalysts within the integrated water–energy–carbon (WEC) sustainability framework. The schematic emphasizes that biochar catalysts should not be developed solely for single-function performance, but rather optimized through a systems-level lens that simultaneously accounts for water remediation efficiency, energy feasibility, and carbon mitigation outcomes. By placing the WEC triad at the center of the framework, Figure 10 highlights the interconnected nature of these domains and illustrates how catalyst design strategies can generate synergistic co-benefits across multiple sustainability targets [85].

3.9.1. Feedstock Alignment and Conversion Pathways

The first design principle emphasizes the importance of aligning feedstock selection and thermochemical conversion routes with nexus priorities. As shown in the schematic, wet wastes and sludge-rich residues are most effectively valorized through hydrothermal carbonization (HTC), which avoids the high energy penalty associated with drying. In contrast, dry agricultural residues are better suited for pyrolysis-based pathways, particularly when coupled with heat recovery from syngas or bio-oil co-products. This rule underscores that upstream feedstock characteristics strongly influence not only the physicochemical properties of the resulting biochar, but also the overall energy and carbon footprint of the production process [66].

3.9.2. Surface Engineering and Catalyst Stability

The second design rule focuses on tailoring surface chemistry and catalyst robustness through heteroatom doping and controlled metal incorporation. The schematic highlights doped carbon frameworks and encapsulated metal active sites as key strategies for enhancing catalytic performance while minimizing environmental risks. Such surface engineering approaches improve conductivity, create defect-rich active centers, and stabilize transition metals, thereby reducing leaching and secondary contamination. This is particularly critical for applications in real wastewater matrices and long-term catalytic cycling, where durability and safety become essential for deployment-ready systems [13].

3.9.3. Selective Pathways in Advanced Oxidation and Catalysis

A major contribution of the nexus framework is the prioritization of selective catalytic pathways under realistic operating conditions. The schematic emphasizes non-radical mechanisms, such as singlet oxygen (¹O₂) generation and surface-mediated electron transfer, which are often more resilient than purely radical-driven processes in complex water environments containing natural organic matter or inorganic scavengers. By promoting selective pathways, biochar catalysts can maintain oxidation efficiency while reducing susceptibility to quenching effects and minimizing undesirable byproduct formation [26,65].

3.9.4. Energy Efficiency and Low-Carbon Processing

The energy dimension of the WEC nexus is represented through the rule of minimizing embodied energy demand during synthesis and operation. Figure 10 highlights low-temperature processing routes and solar-assisted photocatalytic hybrids as promising strategies to reduce energy penalties. These approaches enable catalytic enhancement without relying on aggressive activation procedures or high-temperature treatments, ensuring that water treatment improvements do not impose excessive energy burdens. Such energy-aware catalyst design is particularly relevant for decentralized and low-resource treatment settings [16,66,72].

3.9.5. Carbon Balance and Climate Mitigation Outcomes

Carbon management is presented as a central design pillar through the concept of maintaining a favorable carbon balance. The schematic stresses the need to maximize fixed-carbon yield and long-term stability of biochar while minimizing the lifecycle CO₂ footprint associated with chemical activation or intensive post-modification. This highlights a key nexus trade-off: while aggressive activation may improve catalytic activity, it can also increase embodied emissions and compromise net climate benefits unless performance gains justify the additional carbon cost. Therefore, nexus-aligned catalyst development must integrate carbon efficiency alongside catalytic functionality [60,73,84].

3.9.6. Multifunctionality and Integrated Nexus Benefits

The schematic further reinforces the importance of multifunctionality as a defining advantage of biochar-based catalysts. By combining adsorption, catalytic oxidation/reduction, and carbon sequestration potential within a single material platform, engineered biochars can simultaneously address pollutant removal, renewable energy conversion, and climate mitigation objectives. This multifunctional capacity positions waste-derived biochar catalysts as uniquely suited for circular economy strategies that link waste valorization with sustainable catalysis across interconnected nexus sectors.

3.9.7. Standardized Performance Metrics and Deployment Readiness

Finally, the framework highlights the necessity of standardized evaluation using WEC-relevant performance indicators. Beyond laboratory-scale removal efficiencies, catalyst assessment must incorporate durability in real matrices, regeneration energy demand, metal leaching behavior, and net environmental impacts quantified through techno-economic analysis (TEA) and life cycle assessment (LCA). Such standardized reporting is essential to ensure comparability across studies and to accelerate translation from proof-of-concept demonstrations toward scalable and deployment-ready biochar catalytic technologies [26,27,28,46].

3.10. Reactor Design and Field Performance of Biochar Catalysts

Although biochar-based catalysts have demonstrated strong potential in laboratory-scale studies, their translation into real-world technologies requires addressing key challenges related to reactor integration, operational stability, catalyst regeneration, and system-level sustainability. In practical applications, catalytic performance is strongly influenced by complex wastewater matrices, mass-transfer constraints, catalyst handling, and long-term durability under continuous operation. Therefore, advancing biochar catalysts from proof-of-concept demonstrations toward deployment-ready systems demands greater emphasis on scale-up feasibility, engineered reactor configurations, and lifecycle-oriented performance evaluation within the water–energy–carbon (WEC) nexus.

Most experimental investigations of biochar-catalyzed advanced oxidation processes are conducted in batch reactors under controlled conditions, which often fail to capture the complexity of real treatment environments. In practice, continuous-flow reactor formats are essential for scalable implementation. Packed-bed and fixed-bed reactors represent one of the most realistic pathways for deployment, as biochar catalysts can be immobilized in granular or pelletized form and integrated into column systems for continuous pollutant removal. Such configurations enable simultaneous adsorption and catalytic oxidation while minimizing catalyst loss and simplifying recovery. However, long-term operation in fixed beds may be limited by pore blockage, pressure drop development, and gradual surface fouling, particularly under high organic loading.

Fluidized-bed reactors provide an alternative configuration that improves hydrodynamic mixing and enhances mass transfer between contaminants, oxidants, and catalytic surfaces. By maintaining catalyst particles in suspension, fluidized systems reduce channeling effects and increase contact efficiency, making them attractive for high-throughput industrial wastewater streams. Nevertheless, their practical implementation requires careful control of catalyst attrition, particle stability, and reactor complexity. Beyond conventional reactor engineering, the integration of catalytic biochar into constructed wetlands has emerged as a uniquely sustainable approach. In such hybrid systems, biochar-amended substrates can support combined filtration, adsorption, and reactive oxygen species generation under passive flow conditions. These catalytic wetlands offer strong potential for decentralized and low-energy treatment, aligning closely with circular economy principles and WEC sustainability goals [85,86,87,88].

In parallel, electrochemical reactor architectures represent a rapidly expanding frontier where biochar serves not only as an adsorbent or catalyst support but also as a conductive electrode material. Biochar-based cathodes can promote electro-Fenton reactions, persulfate activation, or electrocatalytic oxidation pathways driven by renewable electricity. Such systems directly connect wastewater remediation with clean energy infrastructure, offering promising opportunities for integrated water–energy solutions [89].

A major requirement for real-world adoption is that biochar catalysts must exhibit durability, reusability, and economically viable regeneration strategies. Catalyst deactivation may occur through pore blockage, surface oxidation, metal leaching in supported systems, or fouling by natural organic matter and inorganic species. Regeneration approaches such as thermal treatment, chemical washing, or electrochemical reactivation can partially restore catalytic activity, but each introduces trade-offs in energy demand, operational cost, and environmental burden. In particular, high-temperature regeneration may compromise carbon stability and increase embodied emissions, while chemical regeneration generates secondary waste streams. Consequently, regeneration pathways must be evaluated through a nexus lens, ensuring that catalytic improvements do not impose excessive energy penalties or undermine net carbon mitigation benefits [22,78,90].

Several deployment-relevant case studies highlight the multifunctional potential of waste-derived biochar catalysts across the WEC nexus. In real wastewater matrices, Fe-loaded and heteroatom-doped biochars have demonstrated resilience against radical scavenging effects caused by dissolved salts, bicarbonate, and natural organic matter. Their effectiveness is often attributed to adsorption–oxidation synergy, whereby pollutants are pre-concentrated on the biochar surface prior to reactive oxygen species–mediated degradation. The incorporation of catalytic biochar into constructed wetlands further enhances long-term removal efficiency by coupling passive filtration with oxidative pathways, providing scalable options for decentralized treatment.

In the energy domain, biochar-derived carbons have emerged as promising alternatives to noble-metal catalysts in ORR systems. Heteroatom-doped biochars exhibit enhanced conductivity, abundant defect sites, and improved oxygen adsorption behavior, enabling competitive electrocatalytic activity in fuel cells and metal–air batteries. Transition-metal biochar composites, particularly Fe–N–C structures, have shown improved durability under alkaline conditions, demonstrating how waste-derived biochar can contribute directly to renewable energy conversion technologies. Biochar-supported catalysts also play an increasing role in carbon management through CO₂ conversion and utilization pathways. Biochar-supported metal nanoparticles, such as Cu, Ni, or Fe, have been explored for CO₂ hydrogenation, dry reforming, and electrochemical CO₂ reduction. The intrinsic alkalinity and surface functional groups of biochar enhance CO₂ adsorption, while strong metal–support interactions improve catalyst dispersion and stability. These systems offer dual carbon benefits by converting greenhouse gases into value-added fuels and chemicals while simultaneously stabilizing biomass-derived carbon within the catalytic framework [37,44,91].

Therefore, these real-world pathways illustrate that biochar-based catalysts are progressing beyond laboratory materials toward scalable platforms for water purification, renewable energy conversion, and circular carbon utilization. However, widespread implementation will require standardized pilot-scale testing in complex matrices, reactor-specific catalyst shaping and immobilization strategies, regeneration protocols with low energy demand, and integrated techno-economic and life cycle assessment frameworks. By addressing these challenges, waste-derived biochar catalysts can evolve into deployment-ready technologies that simultaneously support water security, clean energy transitions, and long-term carbon mitigation within the broader WEC nexus.

4. Challenges and Future Perspectives

Despite the significant promise of biochar-based catalysts as enabling materials within the water–energy–carbon (WEC) nexus, several scientific, technological, and practical barriers must be addressed before their widespread implementation can be achieved. One of the most pressing challenges relates to scalability and techno-economic feasibility. While laboratory-scale investigations consistently report high catalytic efficiencies, translating these outcomes into industrially relevant systems requires optimization of feedstock logistics, process energy demands, and cost competitiveness. In particular, the inherent heterogeneity of municipal and industrial waste streams introduces variability in biochar quality, complicating process standardization, reproducibility, and large-scale manufacturing control.

An additional key obstacle is the attainment of consistent and predictable catalytic activity in real-world treatment environments. The physicochemical properties of biochar catalysts are highly sensitive to precursor composition, pyrolysis temperature, residence time, activation protocols, and post-modification strategies such as heteroatom doping or metal incorporation. Although these factors offer powerful tuning potential, maintaining consistent surface chemistry, porosity, and active site distribution remains challenging. Furthermore, long-term durability is often hindered by catalyst deactivation mechanisms, including pore blockage, surface fouling, oxidative degradation, and metal leaching, particularly in complex aqueous matrices.

From a mechanistic standpoint, an incomplete understanding of structure–property–performance relationships continues to restrict rational catalyst design. Biochar is frequently treated as a “black box” material due to its structural complexity and variability. Therefore, advanced characterization approaches, coupled with computational modeling and in situ spectroscopic techniques, are increasingly necessary to elucidate active site identities, reaction pathways, and synergistic interactions between carbon frameworks and supported catalytic species. Such mechanistic clarity is essential for improving catalyst predictability, selectivity, and application-specific optimization.

Looking forward, several emerging research directions provide strong opportunities for advancement. The development of multifunctional engineered biochars capable of integrating adsorption, advanced oxidation, electrocatalysis, and energy conversion within a single platform is gaining increasing attention. Hybrid architectures incorporating metal oxides, semiconductor interfaces, and single-atom catalytic sites may offer enhanced activity while preserving sustainability. In parallel, exploration of underutilized waste feedstocks that include sewage sludge, mixed municipal organics, and industrial residues can broaden resource recovery pathways and strengthen circular economy integration.

In terms of real-world system deployment, embedding biochar-based catalysts into WEC nexus-driven deployment strategies represents a forward-looking approach to simultaneously address water remediation, renewable energy transformation, and carbon mitigation. Future research should prioritize standardized performance metrics, pilot-scale validation, regeneration and lifecycle durability studies, and comprehensive sustainability evaluation through life cycle assessment (LCA), techno-economic analysis (TEA), and environmental risk frameworks. Ultimately, progress in this field will depend on interdisciplinary collaboration to develop scalable, cost-effective, and environmentally benign biochar catalytic technologies capable of supporting long-term sustainable catalysis within the WEC nexus.

5. Conclusions

This work review positions waste-derived biochar catalysts as a scalable and circular platform for advancing the water–energy–carbon (WEC) nexus through sustainable catalysis. By transforming agricultural residues, industrial by-products, municipal organics, and sludge streams into functional carbon materials, biochar simultaneously supports waste minimization, resource recovery, and carbon retention. The synthesis route and processing routes include pyrolysis, hydrothermal carbonization, and torrefaction—together with physical/chemical activation and post-treatments (heteroatom doping and metal/metal-oxide incorporation), enable controlled tuning of porosity, specific surface area, surface chemistry, conductivity, and the nature of catalytic active sites. These engineered properties underpin biochar’s performance across a wide range of applications, from adsorption–AOP hybrid water treatment and selective radical/non-radical oxidant activation to electrocatalytic energy conversion (HER/ORR/OER), biomass upgrading, and CO₂ utilization. A core outcome of this work is the WEC-aligned design roadmap that links application constraints (matrix complexity, energy availability, and carbon balance) to practical engineering choices such as feedstock alignment, low-energy conversion pathways, stability-focused surface engineering, and selective reaction routes resilient to scavenging in real waters. Importantly, the net sustainability of biochar catalysts must be judged beyond laboratory removal efficiencies by integrating durability, regeneration energy demand, leaching risk, and system-level metrics quantified through techno-economic analysis (TEA) and life cycle assessment (LCA). Despite rapid progress, several barriers remain before widespread deployment, including variability of waste feedstocks, inconsistent reporting of synthesis/activation conditions, limited pilot-scale validation in complex matrices, catalyst shaping/immobilization for continuous-flow reactors, and regeneration strategies that preserve both catalytic activity and carbon benefits. Future research should prioritize standardized performance indicators, mechanistic clarity for active sites and pathways, long-term stability testing under realistic conditions, and integrated reactor-ready designs such as fixed-bed, fluidized-bed, electrochemical systems, and catalytic constructed wetlands. Addressing these challenges will accelerate translation of waste-valorized biochar catalysts into deployment-ready technologies that deliver coupled benefits in water security, clean energy conversion, and carbon mitigation within the WEC nexus.