Advances in Biochar Production and Performance for Sustainable Environment and Energy Applications

Abstract

The urgent demand for sustainable carbon management and environmental remediation has accelerated research on biochar as a multifunctional material. This review critically evaluated over 250 peer-reviewed studies to elucidate the relationships between feedstock composition, thermochemical conversion processes, and the resulting physicochemical properties of biochar. The analysis revealed that pyrolysis temperature is the dominant parameter governing biochar yield and structure, contributing up to ~50% of the variability, while feedstock composition strongly influences surface functionality and pore architecture. Low-temperature biochar (300–400 °C) exhibits higher cation exchange capacity and functional group density, whereas high-temperature biochar (>600 °C) demonstrates enhanced aromaticity, stability, and carbon sequestration potential. Advanced modification strategies significantly improve the adsorption capacity, catalytic activity, and energy applications. Despite these advances, major challenges remain, including lack of process standardization, limited long-term field validation, and uncertainties in carbon stability. This review identifies key research gaps and proposes future directions focusing on scalable production, life-cycle assessment, and integration into circular economy systems, thereby providing a comprehensive framework for the development of high-performance biochar technologies.

1. Introduction

Global agriculture is increasingly threatened by climate change, rising atmospheric carbon dioxide concentrations, prolonged droughts, extreme weather events, and accelerated soil degradation [1,2]. In 2024, for the first time in history, the global temperature crossed the 1.5 °C limit set by the Paris agreement [3]. According to the Intergovernmental Panel on Climate Change (IPCC), global temperatures could rise by an additional 3.2 °C by the end of the century, severely disrupting agricultural systems worldwide [4]. Soil degradation, another consequence of these climatic factors, compromises soil fertility, which is critical for crop growth. These challenges diminish soil organic matter and substantially reduce crop productivity, thereby endangering food security for a rapidly expanding global population. A recent report by the World Bank highlights that these severe socio-economic consequences could push an additional 100 million people into extreme poverty by 2030 [5]. This complex crisis requires innovative, multifunctional strategies that restore soil health and support climate change mitigation through reduced emissions and sustainable resource management [6].

Biochar is a carbon-rich, porous material produced through the thermochemical conversion of organic biomass under oxygen-limited conditions via pyrolysis at elevated temperatures [7]. This engineered form of charcoal is more advantageous for environmental applications, including soil amendment, carbon sequestration, pollution remediation and climate change mitigation. The performance of biochar is closely linked to physiochemical properties that can be tuned through the feedstock type and pyrolysis conditions [8]. Biochar improves soil health in salt-affected soils by enhancing the aggregate stability by 15.0–34.9% and reducing the electrical conductivity through sodium ion adsorption, thereby ameliorating soil salinity stress in agronomic systems [9]. Its ability to change the pH through oxygen-containing functional groups increases the water-holding capacity. This is beneficial for root enlargement with fungal hyphae, creating a more favorable rhizosphere environment [9,10]. These modifications improve plant growth, biomass production, and resilience to abiotic stresses such as drought. Biochar derived from acai agro-industrial waste and applied at 10% (w/w) increased the relative water content in soybean leaves by up to 60%, thereby mitigating the water deficit stress through enhanced stomatal regulation and improved gas exchange [10]. In microbial biofertilizers and biopesticides, biochar serves as a carrier matrix, extending their shelf life. It enhances the nutrient use efficiency by providing protective microhabitats and enabling slow-release mechanisms [11]. For climate mitigation, IPCC considered biochar as a central CO₂ removal strategy, incorporating it into all modeled scenarios aimed at achieving net zero emissions [12]. However, significant uncertainty persists over biochar carbon stability in soils, with persistence estimates ranging from decades to millennia [13]. This underscores the need for harmonized experimental protocols and context-specific calibration. Biochar effectively immobilizes heavy metals in contaminated environments, including lead (Pb), cadmium (Cd), copper (Cu), and arsenic (As), via ion exchange, complexation, precipitation, and electrostatic adsorption [13]. Iron-oxide-, sulfide- or polymer-modified biochar offers enhanced selectivity and sorption, enabling pollutant containment, metal recovery, and circular economy applications [14]. Similarly, it removes emerging contaminants like pharmaceuticals, pesticides, and microplastics from aqueous environments through π-π interactions and pore-filling, lowering their bioavailability and ecological risks [15]. Additionally, microalgae-derived biochar exploits the high photosynthetic efficiency and rapid growth of microalgae to yield nitrogen-rich material ideal for carbon sequestration and slow-release fertilization [7].

Despite rapid progress in biochar research, significant challenges persist that limit its large-scale implementation. These include variability in physicochemical properties arising from diverse feedstocks and process conditions, lack of standardized production protocols, and insufficient long-term field validation of carbon stability and environmental performance. Moreover, existing reviews primarily focus on descriptive summaries rather than establishing integrated relationships between production parameters, structural properties, and application-specific performance.

To bridge these gaps, the present review integrates feedstock, process, property, and application relationships across >250 studies. Novel contributions include an (1) explicit comparison of conventional (slow/fast pyrolysis, gasification) and advanced (microwave-assisted and thermal plasma) thermochemical routes, (2) author-driven synthesis and critical perspective in every subsection rather than simple literature listing, and (3) evaluation of its potential within a true closed-loop circular bioeconomy. As shown in Figure 1, this review outlines the major research trends and structural organization of biochar studies from 2017 to 2026, providing actionable frameworks for researchers and policymakers to accelerate industrial deployment.

Table 1. Critical comparison of recent studies on biochar production from various biomass feedstocks under different pyrolysis conditions, highlighting key physicochemical properties (yield, pH, ash content, and BET surface area), along with identified research gaps and limitations.

No.	Feedstock	Pyrolysis Conditions				Physiochemical Properties
		Type	T (°C)	Yield (%)	pH	Ash (%)	BET (m2 g−1)	EC (mS m−1)	Limitations/Gaps	Ref.
1	Herbaceous and woody plants	Slow	300–700	-	Herbaceous plants 7.68–11.29	Herbaceous plants 21.79–32.71	2.88–301.67 (woody)	6.5–13.2	Lack of standardization of biochar properties across studies	[16]
2	Agricultural waste	Slow	250–350	50% drop	4.0–7.7	39.12	-	56–456.7	Yield–performance trade-off not resolved	[17]
3	Rubberwood sawdust and sewage sludge	Slow	550	-	Rubberwood sawdust 8.41–10.02	Sewage sludge 65.61	2.15–18.42	-	Mechanisms of co-pyrolysis unclear	[18]
4	Bamboo and pigeon pea stalk	Slow	400–600	32.2–27.00	Pigeon pea stalk 5.40	1.58–2.00	16.90–307.10	-	Weak property-application linkage	[19]
5	Pine saw dust, rice husk, food waste, poultry litter, and paper sludge	Slow	350–650		Poultry litter 6.2–10.3	Poultry litter 57.20	Saw dust 3.39–443.79 Rice husk 11.61–280.97	-	No unified evaluation framework	[20]
6	Coconut shells, rice husks, and cattle manure	Slow	300–800	61–68	-	Rice husk 13.38	202.39	-	Energy cost rarely considered	[21]
7	Platanus orientalis L leaf	Slow	500–600	41.2–43	8.4–8.7	-	7.5–8.3	-	Multi-objective optimization lacking	[22]
8	Corn (Zea mays L.) and Conocarpus erectus L. wood	Slow	400–700	Conocarpus wood 63.3	Corn residues 9.56	Conocarpus wood 17.9	Conocarpus wood 167.73	-	Lack of field validation	[23]
9	Biomass waste, wood waste	Slow	350	-	Biomass waste 7.40	Biomass waste 3.63–7.77	19	0.36–0.59	Feedstock effects not fully understood	[24]
10	Sugarcane bagasse, brinjal stem, and citrus peel	Slow	500–600	40.52	6–7	53	360.91	-	Reactor effects not well studied	[25]
11	Catha edulis waste	Slow	350–650	52.26	8.4–11.53	8.8–14.5	221.57	18.24	No predictive models available	[26]
12	Palm kernel shells	Slow	400–800	37.87	-	-	178.13	-	Models lack real validation	[27]
13	Douglas fir wood, Douglas fir bark, and hybrid poplar wood	Fast	623–873	47.9 ± 3	7.9–10.4	Douglas fir bark 2.4	145–500	38	Lifecycle impacts unclear	[28]
14	Pineapple leaf, banana stem, sugarcane bagasse and horticultural substrate	Slow	300–700	Horticultural substrate 50.80	9.69–10.30	13.9	334.67	0.95	Inconsistent experimental results	[29]
15	Wheat straw and wood residue	Slow	350–450	-	8.89	16.59	159.8	216	Limited parameter range studied; broader optimization required	[30]
16	Yak manure	Slow	300–700	41.6	11.8	-	82.9	-	Environmental risks (e.g., salinity, heavy metals) insufficiently evaluated	[31]
17	Waste mushroom substrate	Slow	600–700	-	10.28	54.34	3.36	-	Trade-off between stability and nutrient retention not resolved	[32]

- Denotes the unavailable data.

presents a critical comparison of recent biochar production studies, integrating process conditions, physicochemical properties, and key research gaps to highlight current limitations and the need for more systematic, application-oriented approaches.

2. Biochar Production

Biochar production is influenced by several factors, including the type of feedstock used and the conditions under which pyrolysis is carried out. Different feedstock types, such as woody biomass, crop residues, and animal manure, can impact the final properties of the biochar. Additionally, the pyrolysis method, including variations in temperature, heating rate, and residence time, plays a decisive role in determining the yield and characteristics of the biochar. These factors, along with other process parameters, like atmosphere, pressure conditions, and particle size, collectively influence the efficiency and suitability of biochar for applications in soil enhancement, carbon sequestration, and energy production (Figure 2).

2.1. Feedstock Selection

The choice of feedstock is a key factor determining the biochar yield and characteristics. Agricultural residues constitute a major feedstock category, including wheat straw, rice husks, corn cobs, and lychee waste [33]. Forestry materials such as wood chips, wood bark, and pine wood have been extensively utilized [34]. Waste materials, particularly sewage sludge [35] and food waste, represent promising feedstocks that simultaneously address waste management challenges while producing functional biochar. Higher fixed carbon and lignin content increase char yields. Including fixed carbon and temperature in models markedly improves the prediction accuracy, with experiments confirming a strong fixed-carbon influence [36]. Larger particle sizes slow heat and mass transfer and typically increase the biochar yield, though the precise optimum depends on feedstock (examples: optimal sizes reported near 0.7–2.5 mm in different studies) [37,38]. In this regard, Selvarajoo A. et al. [39] investigated the effect on biochar production of dried citrus peels feedstock via slow pyrolysis. As shown in Figure 3, biochar yields decreased with increasing pyrolysis temperature at 100 °C intervals, declining from 53.62 wt% at 300 °C to 42.32, 31.81, 24.06, and 22.01 wt% at 400, 500, 600, and 700 °C, respectively. In comparison, the corresponding bio-oil yields were 1.72, 4.22, 27.26, 13.92, and 44.88 wt%, while the biogas yields varied between 33.12 and 62.03 wt% across the same temperature range. Results indicate that citrus peel waste is highly recommended to generate a large solid fraction of biochar, particularly at low temperatures.

Sarpong, A.K. et al. [40] investigated the effects of feedstock type (corn cob (CC), coconut husk (CCH), and empty fruit bunch (EFB)) and kiln type (Elsa barrel and top-lit updraft (TLUD) kiln) on biochar (BC) production under slow pyrolysis. They found that pyrolysis temperature increased by 4% and 30% for CC-BC compared with EFB-BC using the Elsa barrel and TLUD kilns, respectively. The BC yields were 71% and 49% higher for EFB using the Elsa barrel and TLUD kiln compared with CC. The Elsa barrel produced an 11% higher burn rate for EFB-BC, while the TLUD kiln showed a 70% higher burn rate for EFB-BC relative to CC. Additionally, CC-BC in the Elsa barrel had a 1.7% higher pH, 4.8% higher EC, 16.8% more TN, and 4.2% higher FC compared with CC-BC in the TLUD kiln. Karlvin et al. [33] compared six different feedstocks (bamboo, dried leaves, wood shavings, rice husk, tree branches, and wood chips) and identified bamboo as the most suitable precursor for water treatment applications, achieving a BET surface area of 317 m²/g and demonstrating favorable adsorption characteristics comparable with commercial activated carbon.

Sun Y. et al. [41] reported that biochar and hydrochar yields strongly depend on the feedstock type. Using hickory wood, bagasse, and bamboo, the biochar yields obtained via slow pyrolysis (300–600 °C) ranged from 22.7 to 43.7 wt%, while hydrothermal carbonization at 200 °C produced higher yields of 27.8–48.4 wt%. Increasing the pyrolysis temperature from 300 to 600 °C consistently reduced the biochar yield due to enhanced thermal decomposition. Among feedstocks, bamboo-derived hydrochar showed the highest yield (48.4 wt%), whereas biochars produced at 600 °C exhibited the lowest yields (~22–26 wt%), highlighting feedstock- and process-dependent optimization of biochar production. From the literature, it was concluded that feedstock lignin content and particle size critically govern biochar yield and quality, yet most studies remain limited to single feedstock evaluations. We argue that systematic multi feedstock comparisons under standardized conditions are urgently needed to bridge this gap and enable regionally optimized production strategies.

2.2. Pyrolysis Conditions and Process Parameters

The pyrolysis temperature emerges as the most critical production parameter affecting biochar characteristics. Studies report pyrolysis temperatures ranging from 300 °C to 850 °C, with distinct property profiles emerging at different temperature ranges [42]. Machine learning and experimental data show a strong negative correlation (Pearson ≈ −0.75) between pyrolysis temperature and yield. Temperature is the dominant factor, explaining ~50% of the modeled importance for yield prediction. Mariyam S. et al. [43] studied the effect of pyrolysis temperature (350–750 °C) on biochar yield governed by feedstock fixed carbon content (14–22%), with a secondary influence from heating rate (5–10 °C min⁻¹). Biochar yields ranged from 23 to 54 wt% with different feedstocks, including date stones, spent coffee grounds, and cow manure. The highest yield (~54 wt%) was obtained at 350 °C and 5 °C min⁻¹, while the lowest (~23 wt%) occurred at 750 °C and 7.5 °C min⁻¹. Increasing the temperature consistently reduced the biochar yield due to enhanced devolatilization, whereas the heating rate showed minimal impact, with yields varying only from 37.27 to 38.02 wt% when increased from 5 to 10 °C min⁻¹ at 550 °C (Figure 4). Experimental studies have reported concrete examples: almond shell yields peaked at 65% at 300 °C and fell markedly at higher temperatures with pilot yields near 39.5% at unspecified higher temperatures, while very high activation temperatures (e.g., 850 °C) produced low yields (~15%) despite high surface areas [38,44]. Several response-surface and optimization studies report higher yields under slow or moderate heating rates (e.g., 5–15 °C min⁻¹) and short-to-intermediate residence times (minutes to a few tens of minutes) depending on feedstock [45,46]. Low heating rates (e.g., 5–15 °C min⁻¹ used in experimental RSM studies) retain more solids versus fast ramps that promote rapid devolatilization and lower yields [26]. Short residence times reduce secondary cracking of volatiles back into gas-phase products, and thus, increase the char yield; optimized yields in experimental studies were obtained with short pyrolysis times (e.g., 5–30 min in sugarcane bagasse and almond shell tests), whereas long residence times (tens to hundreds of minutes) often reduced the yield [47].

Similarly, Premchand P. et al. [48] observed a decrease in biochar yields under slow pyrolysis (400–600 °C) of food waste, rice husk, and grape wood with heating rates of 5–15 °C min⁻¹. However, replacing N₂ with CO₂ consistently enhanced the biochar production. In a CO₂ atmosphere, the biochar yield was increased by up to 4.84% at 400 °C (rice husk), with the enhancement diminishing to ~0.1% at 600 °C, indicating that lower pyrolysis temperatures and heating rates favor carbon retention. These findings highlight CO₂-assisted pyrolysis as an effective strategy for improving biochar yield under controlled thermal conditions. Shen Q. et al. [49] produced biochar via the slow and fast pyrolyses of mallee wood (150–250 µm) at different heating rates (10 °C/s and ∼400 °C/s, respectively). At 500 °C, slow pyrolysis yielded 20.16 wt% biochar, which is higher than fast pyrolysis at the same temperature, producing 16.23 wt%. During subsequent rapid pyrolysis at 1300 °C, char yields from both biochars decreased slightly from ~77% to ~75% as the residence time increased from 0.45 to 1.4 s. However, fast-pyrolysis biochar showed marginally higher early-stage yields due to a higher ash content.

This implies that slow pyrolysis enhances the initial biochar yield, while the heating rate has a limited impact on char stability under high-temperature conditions. Potnuri R. et al. [50] reported that biochar yield was significantly enhanced by torrefaction of sawdust followed by microwave-assisted pyrolysis. Torrefaction at 125, 150, and 175 °C produced 24–48 wt% biochar however, the yield decreased with increasing catalyst loading (5–15 g KOH). Using 20 g of feedstock and 5 g KOH, the biochar yield increased from 39.2 wt% (125 °C) to 41.9 wt% (150 °C) and 48.1 wt% (175 °C), whereas it declined to 24.2–35.0 wt% at 15 g KOH. The highest yield (48.1 wt%) was obtained at 175 °C with 5 g KOH, while the lowest (24.2 wt%) occurred at 125 °C with 15 g KOH. While the pyrolysis temperature predominantly governs biochar yield and properties, while other parameters show secondary effects. However, the lack of integrated and scalable optimization limits predictive control, necessitating coupled process design approaches.

2.3. Advanced Pyrolysis Techniques

Beyond conventional methods, advanced thermochemical technologies like microwave-assisted pyrolysis (MWP) and thermal plasma pyrolysis (TPP) have emerged to optimize biochar properties through non-traditional heating mechanisms. Recent studies have utilized MWP at temperatures ranging from 250 to 900 °C, with power levels typically between 100 and 900 W [51]. According to Khelfa et al. [52], microwave power between 350–550 W can reduce residence times to 10–20 min, while Fodah & Abdelwahab, [53] identified an optimal power of 700 W (integrated with 10 wt% activated carbon) for maximizing the efficiency in solar-powered MWP systems. TPP operates at extreme thermal regimes, often exceeding 1000 °C, using plasma torches or radiofrequency (RF) systems. Muvhiiwa et al. [54] and Tang & Huang [55] employed nitrogen and air plasma torches with power inputs ranging from 0.1 kW to 20 kW. Their results show that these high-energy environments facilitate near-complete decomposition, producing biochar with a carbon content as high as 93% at 1000 °C. Despite their advantages, these advanced techniques face challenges related to energy efficiency, scalability, and economic feasibility.

3. Physicochemical Properties

The physicochemical properties of biochar include the surface area, porosity, surface functional groups, pH, electrical conductivity (EC), and cation exchange capacity (CEC), which are fundamentally important as they dictate its efficacy across various agricultural and environmental applications (Figure 5). These properties directly influence biochar ability to improve soil health, enhance crop productivity, mitigate climate change through carbon sequestration, and remediate contaminated environments [15,56,57]. A high pH in biochar is advantageous for ameliorating acidic soils [58], while a large surface area and porous structure are crucial for its function as a sorbent for pollutants [56]. A high CEC resulting from negatively charged surfaces promotes nutrient retention and soil fertility by reducing Ca, K, and Mg leaching [56]. Furthermore, carbon stability determines biochar persistence and sequestration potential in soils. Therefore, understanding and manipulating these properties are key to optimizing biochar for specific purposes.

3.1. Structural Properties and Pore Architecture

Biochar pore structures vary widely, with specific (BET) surface areas ranging from <1 to several hundred m² g⁻¹ and pore sizes spanning macropores (µm) to micro-/mesopores (nm), depending on the feedstock and pyrolysis conditions. The temperature dependence of a BET surface area is central to the adsorption capacity and varies strongly with both feedstock and pyrolysis conditions. N₂ adsorption–desorption hysteresis patterns (H4 type) indicated combined micro/mesoporous structures in some high-temperature chars [59]. Kuo L.A. et al. [60] examined the textural properties of biochar produced by microwave-assisted pyrolysis of rice husk (RH) at temperatures > 450 °C using output powers of 300–1000 W and residence times of 5–15 min. According to the results, pore development markedly improved with increasing microwave power (300 → 1000 W) at a residence time of 5 min, achieving the highest BET surface area (172.04 m²/g) and total pore volume (0.1229 cm³/g) at 1000 W for 5 min. Prolonged residence times (10–15 min) at 1000 W diminished these values. N₂ adsorption desorption isotherms (77 K) displayed hybrid Type I/IV behavior with H4 hysteresis, signifying slit-shaped micropores and mesopores. BJH analysis indicated a mesopore peak at ~3.8 nm, while Horváth–Kawazoe evaluation of the optimal biochar revealed dominant micropores at ~0.6 nm. SEM micrographs showed increased surface roughness and porosity after pyrolysis due to thermal decomposition of the biomass structure (Figure 6). Li H. et al. [61] utilized machine learning to predict and optimize the specific surface area (SSA) and total pore volume (TPV) of biochar derived from biomass pyrolysis based on a dataset of 169 samples incorporating biomass proximate/elemental compositions and pyrolysis parameters. Gradient-boosting regression (GBR) achieved superior performance over random forest, with test R² values of 0.89–0.94. Feature importance analysis identified pyrolysis temperature, biomass ash content, and volatile matter as the primary determinants of SSA and TPV. Multi-target GBR optimization of pyrolysis conditions and binary biomass mixing ratios facilitated experimental validation of high-SSA/TPV biochar, underscoring the machine learning efficacy for the targeted engineering of biochar textural properties in adsorption and porous carbon applications.

Lu S. et al. [62] characterized pore structures of biochar from different feedstock types (herbaceous, coniferous, and broad-leaf biomass) pyrolyzed at 550 °C using nitrogen adsorption and mercury intrusion porosimetry (MIP). According to the findings, the BET specific surface area (SSA) ranged from 1.06 to 70.22 m²/g (mean 10.61 m²/g), exposing primarily nanoscale pores (<0.1 μm). The total pore volumes revealed by MIP were 1.28–3.68 cm³/g, with porosities of 57.8–79.7%, while herbaceous biochars (HBs) exhibited a higher average total pore volume and porosity, among others. The MIP pore size distribution was bimodal in the micrometer range (peaks ~1.5–5 μm and 5–25 μm), dominated by storage pores (0.5–50 μm, ~85% of volume). The feedstock type had a greater influence on the pore characteristics than pyrolysis conditions (350–550 °C, 2–5 h). These hierarchical pores support biochar functions in soil water storage, microbial habitats, and pollutant sorption. Biochar pore structure is governed by feedstock and pyrolysis conditions. While limited understanding of structure-property relationships hinders its targeted design for specific applications.

3.2. Surface Chemistry and Functional Groups

Biochar surfaces present a complex ensemble of oxygenated, nitrogenous and sulfur-bearing moieties that arise during thermochemical conversion and subsequent oxidation; these groups determine chemical reactivity toward metals, organics and nutrients. Carboxyl, hydroxyl (alcohol/phenolic), carbonyl (quinone/ketone), lactonic and ether functionalities are commonly reported on biochar surfaces and are collectively termed OCFGs in recent reviews [63]. Phenolic -OH bound to aromatic rings and conjugated carbonyls frequently occur on lower-temperature chars and after surface oxidation. Pyridinic, pyrrolic, amine and amide-type N functionalities occur when N-rich feedstocks or amination treatments are used and influence basicity and coordination chemistry [64]. Thiols, sulfides, sulfonic and sulfate species appear mainly from S-containing feedstocks or sulfonation treatments and can introduce redox and acid sites. Quinone-like and phenoxyl structures contribute to electron transfer and catalytic redox behavior on biochar surfaces [65].

Barszcz W. et al. [66] demonstrated that the surface functional groups of biochar are highly dependent on pyrolysis temperature. Low-temperature biochar (~300 °C) exhibits abundant oxygen- and hydrogen-containing functionalities, such as -OH, C=O, and C-O groups derived from cellulose and hemicellulose decomposition; with increases in pyrolysis temperature above 600 °C, these functional groups progressively diminish due to dehydration and deoxygenation reactions, accompanied by enhanced aromatization and the formation of condensed polycyclic aromatic carbon structures (Figure 7a).

Luo Q. et al. [67] investigated the effect of pyrolysis temperature (250–700 °C) on the surface chemistry of corn-stover-derived biochar. The surface carbon content increased from 63.10% to 80.58% by increasing the temperature, while the oxygen content decreased from 26.42% to 9.20%, along with an increase in biochar pH from 6.60 to 10.66. XPS spectra showed a pronounced enhancement of the C1s signal from 79.42% to 91.02% and a subsequent decrease in the O1s signal from 20.58% to 8.98%. Deconvolution of C1s and O1s peaks revealed an increasing proportion of C-C and O=C-O species, whereas oxygenated functional groups such as O-H, C-O-C, and C-OOH declined markedly with temperature. These results indicate progressive dehydration, deoxygenation, and enhanced surface aromaticity, consistent with FTIR and Boehm titration results.

Elnour A.Y. et al. [68] produced date-palm-biomass-derived biochar at 300–700 °C and investigated the evolution of surface functional groups. At low temperatures, FTIR spectra show a strong band at ~3394 cm⁻¹ associated with -OH stretching, as well as aliphatic C-H vibrations at ~2927 cm⁻¹. With an increase in temperature, the spectra gradually weakened and became negligible at 700 °C due to intensified carbonization and volatile removal. Carbonyl (C=O, ~1693 cm⁻¹) and aliphatic C=C (~1597 cm⁻¹) vibrations also decreased markedly with temperature, indicating cracking of aliphatic chains and deoxygenation reactions. At temperatures above 600 °C, most aliphatic functional groups disappeared, accompanied by the formation of more condensed aromatic and graphite-like structures, consistent with reduced O/C and H/C ratios and diminished FTIR peak intensities (Figure 7b). Biochar surface functional groups decrease with higher pyrolysis temperatures, increasing the aromaticity. However, systematic studies linking chemistry to reactivity are limited.

Figure 7. FTIR spectra showing the effect of pyrolysis temperature on surface functional groups of biochar. (a) Biochar produced at 300–800 °C showing the progressive loss of oxygenated functional groups with increasing temperature; reproduced with permission from [66]. (b) Date palm biomass derived biochar at 300–700 °C, highlighting the decreases in -OH (3394 cm⁻¹), C-H (2927 cm⁻¹), and C=O (1693 cm⁻¹) groups along with the formation of aromatic structures; reproduced with permission from [68].

Figure 7. FTIR spectra showing the effect of pyrolysis temperature on surface functional groups of biochar. (a) Biochar produced at 300–800 °C showing the progressive loss of oxygenated functional groups with increasing temperature; reproduced with permission from [66]. (b) Date palm biomass derived biochar at 300–700 °C, highlighting the decreases in -OH (3394 cm⁻¹), C-H (2927 cm⁻¹), and C=O (1693 cm⁻¹) groups along with the formation of aromatic structures; reproduced with permission from [68].

3.3. pH, Ash Content, and Ion Exchange Behavior

Surface charge and pH behavior link functional group acid–base chemistry, ash/mineral content, and the point of zero charge to sorption outcomes and electrostatic interactions. Many chars are alkaline in bulk aqueous suspension because of mineral ash (carbonates, oxides) that impart basicity, although acid biochar can be produced by oxidation or acid treatments [64]. The pH_pzc of biochar varies widely with the feedstock, temperature and modification; surface acidity from oxygenated groups lowers pH_pzc, while ash/basic minerals raise it, causing pH_pzc shifts that alter whether the surface is net positive or negative at a given solution pH [69]. The surface charge as measured by the zeta potential tracks pH and surface group deprotonation; deprotonation of carboxyl and phenolic groups produces negative charge and promotes cation uptake.

Furthermore, alkaline conditions in biochar systems promote lignocellulosic breakdown via cleavage of ester and ether linkages, enhancing solubilization of organic matter, while acidic conditions favor proton-driven hydrolysis of hemicellulose. Moreover, pH regulates microbial and enzymatic activities, thereby controlling the decomposition rates and nutrient mineralization. Thus, biochar-induced pH governs both sorption behavior and biomass degradation pathways. He et al. [70] meta-analyzed 465 studies on the biochar mitigation of soil greenhouse gas (GHG) emissions and nutrient losses, complemented by DFT calculations, to assess feedstock (straw/SB, wood/WB, other/OB) and pyrolysis temperature effects on properties relevant to pH, ash, and ion exchange. Wood-based biochar showed superior porosity and specific surface area, while straw-based/low-temperature biochar retained higher O, N, and H contents and functional groups (e.g., -OH, -COOH, -NH₂), enhancing the surface charge density, charge transfer, bonding orbitals, active adsorption sites, and adsorption energy/stability for soil ions/leachates (e.g., NH₄⁺, NO₃⁻). The pH increased with temperature (e.g., ~9.424 > 800 °C for SB; overall SB highest at 9.261 vs. WB 8.695), the ash content was the highest in OB (31.053%) vs. WB (15.236%), and the cation exchange capacity (CEC) decreased with temperature (highest 55.717 cmol kg⁻¹) (Figure 8A).

Pantoja et al. [71] examined non-modified banana leaf biochar produced by slow pyrolysis at 300 °C, 400 °C, and 500 °C. Higher pyrolysis temperatures led to elevated ash levels (from mineral enrichment as volatiles escaped) and a pH increase from ~8.0 (at 300 °C) to ~9.5 (at 500 °C), driven by alkaline ash accumulation and deprotonation effects. This dramatic increase in pH was due to enhanced alkalinity from alkaline ash accumulation and deprotonation of residual functional groups. Zeta potential measurements (at 0.01 M NaCl) showed more negative surface charge at lower pyrolysis temperatures (e.g., reaching ~−50 to −55 mV in the pH 6–10 range for 300 °C biochar), facilitating stronger electrostatic attraction and cation exchange for NH₄⁺, whereas higher-temperature biochar exhibited a less negative charge and reduced ion retention capacity.

This contributed to the highest NH₄⁺ adsorption (7.0 mg/g) at 300 °C, decreasing to 6.1 mg/g (400 °C) and 5.6 mg/g (500 °C), which is linked to preservation of oxygen-containing functional groups at lower temperatures that support surface charge and ion exchange sites. Equilibrium data fitted the Harkins–Jura isotherm best, indicating multilayer adsorption on heterogeneous surfaces influenced by these charge/pH dynamics (Figure 8B). Nguyen et al. [72] prepared sugarcane bagasse biochar (SB-BC) via H₃pO₄ pretreated with slow pyrolysis and modified with KOH to produce oxygen-containing functional groups (OCFGs) for enriched NH₄⁺ adsorption from aqueous solutions. According to the findings, the OCFGs increased around fourfold after the KOH modification, boosting the maximum Langmuir adsorption capacity from 27.1 mg/g (SB-BC) to 53.1 mg/g (SB-MBC). Optimal NH₄⁺ uptake occurred at pH 7.0 using 3.0 g/L of biochar and 50 mg/L initial NH₄⁺ at 25 ± 2 °C over a 180 min equilibration period. The adsorption process followed Elovich kinetics, with equilibrium data fitting the Langmuir and Sips isotherm models best, consistent with monolayer chemisorption as the primary mechanism. Mechanisms included cation exchange, electrostatic attraction, and surface complexation, as confirmed by pre- and post-adsorption characterizations. Biochar alkalinity and ash content drive the surface charge and cation exchange, but their effects are feedstock- and temperature-dependent. More comparative studies are needed to optimize these properties for targeted adsorption.

4. Applications of Biochar

Biochar has emerged as a highly versatile material with a wide range of applications in environmental management, agriculture, and sustainable development, as illustrated in Figure 9. The tunable surface chemistry, porous structure, high surface area, and long-term stability of biochar make it suitable for multiple applications. In the agriculture sector, it enhances soil fertility and crop resilience through enhanced water retention, nutrient availability, and pH buffering [73]. In climate mitigation, it works for carbon sequestering by storing recalcitrant carbon in soils [74,75], removing contaminants (heavy metals, ammonium, organics) from water and soil via adsorption, ion exchange, and complexation [76,77]. It supports waste valorization and energy co-production, acting as a carrier in biofertilizers and animal feed additives [78], thus contributing to construction materials and circular economy strategies [79]. These diverse functions position biochar as a key tool for addressing soil degradation, food security, water quality, and net-zero emissions goals in a changing climate.

4.1. Agricultural Applications and Soil Amendment

In agricultural systems, biochar amendment has been widely investigated for its potential to improve soil structure, water retention, and overall soil health. Yuan et al. [9] conducted a global meta-analysis to quantify the effects of biochar amendment on the health of salt affected soils. They reported that with the addition of biochar, soil physicochemical properties, including aggregate stability (15.0–34.9%), porosity (8.9%), and water retention capacity (7.8–18.2%), were significantly enhanced by increasing the cation exchange capacity (21.1%), soil organic carbon (63.1%), and nutrient availability (31.3–39.9%), while the bulk density (6.0%) and salt stress alleviation (4.1–40.0%) were decreased. Following biochar incorporation, soil biological health can also be improved, particularly enhancing microbial biomass (7.1–25.8%), facilitating enzyme activity (20.2–68.9%), and ultimately increasing plant growth, as shown in Figure 10a. Farhangi-Abriz S. et al. [80] reported that biochar application significantly increased the grain yields in maize (by 14–35%) and wheat (by 13.5%), with the greatest improvements observed in coarse-textured and acidic soils. Biochar derived from animal waste outperformed that from crop residues and wood waste as a feedstock. Yield enhancements were more pronounced under subtropical climates, whereas high application rates (>30 t ha⁻¹) did not substantially affect the crop yield.

According to Wu B. et al. [81], biochar is an effective soil amendment for salt-affected soils, with a meta-analysis showing crop productivity increases of 13–37% and soil salinity changes ranging from −20% to 20%. Optimal improvements in productivity were achieved with biochar applied at 40–50 t ha⁻¹, with a C/N ratio of 40–60, pyrolysis temperatures of 450–550 °C, and pH 7–8. Meanwhile, reductions in soil salinity were associated with biochar having an EC < 2 mS cm⁻¹, application rates of 20–30 t ha⁻¹, higher C/N ratios (>80), or pyrolysis temperatures < 450 °C. The strongest combined benefits for crop productivity and salinity management were observed in sulfate-dominated saline soils, particularly under salinity > 3 g kg⁻¹ and soil pH 7–9, highlighting the importance of tailoring biochar properties to soil constraints (Figure 10b). Han H. et al. [82] conducted 381 datasets from 63 studies and reported biochar application as a soil amendment increased crop yield by 14.45%, water use efficiency by 14.28%, and nitrogen use efficiency by 13.97%. The efficacy of biochar was dominated by soil organic carbon, total nitrogen, bulk density, biochar total carbon content, and C:N ratio. Optimal synergistic improvements in crop productivity and resource use efficiency were achieved using biochar with lower carbon content (TC ≤ 200 g kg⁻¹, C: N 50–100). These effects were most pronounced in compacted soils with high bulk densities (>1.4 g cm⁻³) and low nutrient levels (SOC 10–15 g kg⁻¹, TN ≤ 0.5 g kg⁻¹). Overall, the results offer practical guidance for tailoring biochar application strategies to specific soil conditions in agricultural systems.

Sewage sludge biochar application improved the soil pH from 5.8 (control) to 6.5 (2% biochar) in strongly acidic soils, demonstrating its liming effect. This pH amelioration is particularly valuable for acidic soils, where aluminum toxicity and nutrient deficiencies limit crop production [74]. de Sousa Lima J.R. et al. [83] reported a 29% increase in soil water content in Regosol soils of the Caatinga biome following biochar application, demonstrating its potential for improving water availability in semi-arid regions. This enhanced water retention stems from porous structure and high surface area of biochar, which create additional water storage sites within the soil matrix. The total organic carbon (TOC) increased from 5.5 g kg⁻¹ to 6.2 g kg⁻¹ following biochar application in Regosol soils. Shi S. et al. [84] provided long-term evidence from a decade-long field experiment, statistically verifying that consecutive biochar application greatly enhanced soil organic carbon (SOC) sequestration, building upon earlier 4-year experiments showing significant SOC increases with wood biochar.

According to Iqbal M. et al. [85], wood bark biochar demonstrated particularly strong effects on maize growth, increasing the plant height by 48.75% and chlorophyll content by 21.07%. The study also documented enhanced nutrient uptake, with N, P, and K uptakes increasing by 21.6%, 31.25%, and 45%, respectively, compared with control treatments.

Madari et al. [86] investigated biochar effects on soybean grain yield in a five-year field trial on the sandy clay loam Haplic Ferralsol, providing valuable long-term agricultural performance data. Strock J. et al. [87] found that biochar alone removed 86% of NH₄⁺ and 77% of NO₃⁻ from aqueous solution, though only 52% and 33% showed significant sorption isotherms, respectively. When mixed with soil, only 18% of biochar increased the NH₄⁺ adsorption, and none increased the NO₃⁻ removal, indicating that biochar–soil interactions can reduce the nutrient removal efficiency. In the literature, the optimal effects of biochar depends on the feedstock type, pyrolysis temperature, and application rate, with animal waste biochar often outperforming crop residue or wood-derived biochars.

4.2. Water Treatment and Pollutant Removal

Recent studies highlight the effectiveness of biochar in pollutant removal from contaminated environments, driven by its high surface area and abundant functional groups. Beigmohammadi F. et al. [88] worked on grape-residue-derived biochar and found it highly effective for immobilizing potentially toxic elements due to its high surface area and functional groups. Contaminated soils samples were collected from agricultural, urban, and industrial lands and biochar was applied at 5% (w/w). Biochar reduced the diethylenetriamine-penta-acetic-acid-extractable Cu, Ni, Pb, and Co concentrations from 7.26 to 5.54 mg/kg, 1.83 to 0.86 mg/kg, 5.82 to 4.06 mg/kg, and 0.25 to 0.18 mg/kg, respectively, corresponding to reductions of 24–79% after two months of incubation. The study, based on 110 surface soil samples and supported by high characterization techniques, highlighted that metal immobilization was primarily controlled by negatively charged functional groups, the high specific surface area, and improvements in soil organic matter and microbial respiration. This study highlights biochar-based materials for heavy-metal removal applications in water and wastewater treatment systems. Piscitelli L. et al. [78] demonstrated that biochar-amended green roof substrates enhance the retention of pollutants in urban runoff, reducing contaminant loads to wastewater systems. Column experiments with biochar from olive husks (450 °C) and forest waste (850 °C) mixed with peat or volcanic rock showed superior sorption of phenanthrene and heavy metals compared with conventional substrates.

El Barkaoui S. et al. [89] used lab-scale column filtration systems to evaluate the performance of biochar-based filtration systems for wastewater treatment, incorporating biochar at 0%, 10%, 25%, and 50%. Biochar produced from exhausted olive pomace (temperature 590 °C, time 2 h, heating rate 10 °C min⁻¹) significantly enhanced the pollutant removal. The total removal efficiencies were 64–65% for nitrogen, 75–77% for Kjeldahl nitrogen, 39–44% for phosphorus, 78–87% for organic nitrogen, 44–56% for total chemical oxygen demand, and 33–51% for chemical oxygen demand. Meanwhile, 57–69% for NH₄⁺-N, 38–42% for PO₄³⁻, and 87–92% for total suspended solids were observed. Microbial indicators including total and fecal coliforms, fecal streptococci, Staphylococcus, and total aerobic mesophilic flora were most effectively removed at a biochar addition rate of 10%. This study concluded that a low biochar content (10%) provides optimal wastewater treatment performance and economic feasibility (Figure 11a).

Jenjaiwit S. et al. [91] investigated the removal of triclocarban (TCC), an emerging endocrine disruptor, from municipally treated wastewater. Pseudomonas fluorescens MC46 was identified as an effective TCC-degrading bacterium for wastewater treatment. The study evaluated microbial cells immobilized on waste biochar (derived from wood vinegar production) as a highly effective advanced wastewater treatment unit for TCC removal. Wastewater samples were treated using biochar alone, cell-immobilized biochar, and free bacterial cells, under batch (short-term) and semi-batch (long-term) operational modes. In the semi-batch system, cell-immobilized biochar achieved high TCC removal efficiency (up to 65%). Furthermore, reused cell-immobilized biochar exhibited more stable TCC removal performance than biochar alone and free MC46 cells. Sonsuphab K. et al. [90] also investigated the remediation of TCC-contaminated water using biochar-immobilized bacterial cells. After long-term operation (160 days), TCC removal performance was significantly improved. The TCC removal efficiency of biochar-immobilized cells reached 84–97%, whereas degradation by free Pseudomonas fluorescens strain MC46 was 76–94%. MC46 produced extracellular polymeric substances (EPSs), which enhanced TCC adsorption. Cell shrinkage further improved TCC resistance, contributing to effective water detoxification. However, after 100 days of operation, detachment of MC46 cells from the immobilized cell column was observed. The study concluded that emerging contaminants such as TCC can be removed efficiently and economically in a manner consistent with sustainable development through the value-added utilization of waste materials (Figure 11b). High surface area and functional groups lead to biochar providing effective heavy-metal and organic pollutant removal, while biochar-amended systems and immobilized microbes enhance the long-term wastewater treatment efficiency.

4.3. Nutrient Management and Greenhouse Gas Mitigation

Modified chars have shown enhanced nutrient adsorption capabilities, making them promising materials for nutrient management and emission control strategies. In a study by Kang J.K. et al. [92] synthesized Fe-loaded food waste biochar (Fe-FWB) via blending food waste with Fe (0.1, 0.3, and 0.5 M) and optimized the pyrolysis time and temperature (2–4 h, and 300–600 °C) using the response surface methodology. Phosphate uptake followed pseudo-second-order and Elovich kinetic models, indicating diffusion and surface-controlled sorption. Phosphate adsorption was better described by Freundlich and Redlich–Peterson isotherms than the Langmuir model. Thermodynamic analysis revealed positive ΔG⁰ values, indicating non-spontaneous adsorption, while positive ΔH⁰ and ΔS⁰ values confirmed an endothermic and entropy-driven process. Phosphate adsorption decreased as the pH increased from 3 to 11 and was inhabited by coexisting anions (HCO₃⁻, SO₄²⁻, and NO₃⁻), demonstrating the potential of Fe-FWB as a sustainable adsorbent for phosphate removal (Figure 12a). Liao X. et al. [93] applied three-year aged biochar at 12 t/ha on sandy loam soil in the North China Plain. This study influenced fertilizer nitrogen retention and N₂O emissions during the maize growing season. According to the results, maize took up roughly 25.6–26.2% of the applied urea nitrogen as ¹⁵N-labeled urea in microplots, while biochar had no noticeable impact on the crop yield or overall fertilizer nitrogen recovery. In the top 40 cm of soil, the residual fertilizer nitrogen was similar between treatments (around 20–22%), while biochar increased the retention by about 10% in the surface 0–20 cm layer and reduced it by 37% in the 20–40 cm layer. Noticeably, N₂O emissions dropped from 2.06 to 1.89 kg N ha⁻¹ and fertilizer-derived N₂O from 0.78 to 0.74 kg N ha⁻¹ with biochar addition. Fertilizer nitrogen contributed 38–39% of the total N₂O, while priming soil organic nitrogen mineralization accounted for 34% under control conditions and 30% with biochar (Figure 12b).

Zhang C. et al. [94] evaluated the effects of biochar amendment (0, 2, 4, and 6 kg m⁻²) on nitrogen (N) and phosphorus (P) loss in barren karst yellow soil using indoor leaching simulations and field runoff observations under different rainfall intensities. The impact of biochar application on oil peony growth was further assessed through comparative growth experiments. Biochar application increased the soil pH, total N, total P, and available nutrients while reducing nutrient leaching, with 2 kg m⁻² achieving the lowest losses of TP, TN, nitrate N, and ammonium N in leachate. Under natural rainfall, 4 kg m⁻² biochar most effectively reduced the TP, TN, and ammonium N losses in surface runoff, whereas the nitrate N loss remained high under rainstorm, heavy, and moderate rainfall events. Biochar at 4 kg m⁻² (fertilized soil) and 6 kg m⁻² (unfertilized soil) most effectively enhanced the plant height, basal diameter, leaf SPAD, and tissue N and P contents, demonstrating that biochar improves soil fertility, mitigates N and P losses, and promotes oil peony growth in karst systems (Figure 12c). Zhao Y. et al. [95] conducted a two-year experiment on biochar for the reduction of pollutants in agricultural runoff at various temperatures (350–650 °C) for the enhancement of soil fertility. The results show that the biochar significantly reduced the concentrations of total nitrogen and total phosphorus in farmland runoff. Among rice-straw-based biochar (RB350 °C, and RB650 °C), RB650 exhibited the best performance, with total reductions of nitrogen and phosphorus output loads by 29.31–30.67% and 21.92–25.21%, respectively. Higher-temperature biochar is more conducive to the accumulation of soil nutrients. This was accompanied by higher soil organic matter; increased nutrient availability; and significant (p < 0.05) increases in pH, organic matter, total nitrogen, and total phosphorus in the second year after biochar application. In a recent study by Brtnicky M. et al. [96], all soil amendments positively influenced soil health parameters, including soil enzymes and basal and substrate-induced respiration, leading to enhanced nutrient-acquiring enzymes (C, N, and P), improved soil fertility, and increased lettuce biomass. Although the plant nutrient content was largely determined by mineral fertilization, activated biochar also contributed to higher nutrient accumulation in the biomass. Modified biochar improves nutrient retention, reduces nitrogen and phosphorus losses, and lower N₂O emissions, supporting sustainable soil and climate management.

4.4. Energy Production

The utilization of biochar as a support material in energy production and storage systems has emerged as a promising strategy to enhance thermal performance. Gowthami D. et al. [97] developed leak-resistant form-stable phase-change materials (FSPCMs) for solar energy storage using date seed biochar as a porous support matrix. Date seed biochar (DB) and chemically activated biochar (ADB) were prepared via pyrolysis at 550 °C, exhibiting BET surface areas of 82 and 197 m²/g, respectively. Both chars were used to encapsulate eutectic phase-change material (PA-PEG6000), with ADB showing a higher PCM loading capacity due to its larger surface area. The physical integration of biochar and PCM was confirmed by FTIR, XRD, and FESEM analyses. The ADB/PA-PEG6000 composite exhibited a higher latent heat (210.51 J/g) and melting temperature (56.4 °C) than the DB-based composite (202.43 J/g, 55.3 °C), while maintaining thermal and chemical stability over 500 thermal cycles, demonstrating strong potential for solar water- and air-heating applications (Figure 13A).

Patwa D. et al. [98] optimized biochar production for engineered thermal backfill applications in crude oil pipelines designed for 25–50 years of service life. Water hyacinth and sugarcane bagasse feedstocks were used to produce biochar via pyrolysis at 300–700 °C. The biochar was then evaluated for thermal conductivity, energy consumption, yield, and soil stability. The thermal conductivity of all samples ranged narrowly between 0.10 and 0.13 W m⁻¹ K⁻¹, which implies that the pyrolysis temperature has a limited effect on the heat insulation performance. The lowest energy consumption and highest yield were obtained for biochar produced at 300 °C, while it exhibited insufficient carbon stability in soil. A higher energy efficiency of 60% was achieved at 400 °C among all temperatures while meeting the stability requirements for long-term thermal backfill applications.

Figure 13. (A) Form-stable phase-change materials based on activated date seed biochar for solar thermal energy storage; reproduced with permission from [97]. (B) Current–voltage and current–power density curves (a), with performance comparison of DC-SOFCs powered by different carbon fuels (b); reproduced with permission from [99]. (C) Techno-economic and sensitivity analysis of a biochar-based slurry fuel production plant; reproduced with permission from [100]. Note: Figure enhanced via AI (Gemini 3 Pro, April 2026). Final content is author-validated.

Figure 13. (A) Form-stable phase-change materials based on activated date seed biochar for solar thermal energy storage; reproduced with permission from [97]. (B) Current–voltage and current–power density curves (a), with performance comparison of DC-SOFCs powered by different carbon fuels (b); reproduced with permission from [99]. (C) Techno-economic and sensitivity analysis of a biochar-based slurry fuel production plant; reproduced with permission from [100]. Note: Figure enhanced via AI (Gemini 3 Pro, April 2026). Final content is author-validated.

Miao K. et al. [99] evaluated crop-straw-derived biochar from sesame and eggplant straw as fuels for direct carbon solid oxide fuel cells (DC-SOFCs) at 850 °C. The sesame straw biochar achieved a maximum power density of 228 mW cm⁻² and an operational lifetime of 30.7 h with 34.4% fuel utilization at 50 mA, while eggplant straw biochar delivered 214 mW cm⁻², a 21.1 h lifetime, and 23.6% fuel utilization, both outperformed activated carbon (173 mW cm⁻²). The enhanced DC-SOFC performance was attributed to the porous structure, higher surface area, disordered carbon, and inherent metal catalysts of biochar, which promoted the reverse Boudouard reaction and improved the cell stability and efficiency (Figure 13B). Zepeda L.C. et al. [100] assessed the economic feasibility of producing biochar-based slurry fuels from rice straw at processing capacities of 10 and 40 t h⁻¹. For slurries containing 40 wt% biochar, the minimum selling prices of water-based (CS-W100) and ethanol-based (CS-E100) fuels were A$ 0.07 and A$ 0.052 MJ⁻¹, respectively, with the most cost-effective scenario achieved at 40 t h⁻¹ producing CS-E100 at A$ 1434.30 t⁻¹ (A$ 0.052 MJ⁻¹). A sensitivity analysis showed that increasing CS-E100 prices to A$ 1476.04 and A$ 1566.63 t⁻¹ resulted in payback periods of 14 and 8 years and internal rates of return of 14% and 22%, respectively. A Monte Carlo analysis indicated 74.73% and 98.79% probabilities of positive net present value (A$ 44.44 and 140.90 million), demonstrating the potential of biochar slurry fuels as economically viable waste to energy solutions (Figure 13C). Uddin M.M. et al. [101] assessed the economic and environmental benefits of adding corn stover biochar to anaerobic digestion for renewable natural gas (RNG) production from beef manure. Process modeling showed that biochar increased the biogas yield and reduced the minimum RNG selling price by 15%, with additional reductions of up to 22% when digestate revenue and Renewable Identification Number (RIN) credits were included. Life cycle assessment indicated a 32% decrease in greenhouse gas emissions, mainly due to avoided manure emissions and carbon sequestration when biochar-amended digestate was applied to soil. Tsui T.H. et al. [102] examined biochar-enabled direct interspecies electron transfer (DIET), showing it can improve the sustainability of anaerobic digestion by reducing hydrogen sulfide formation in sulfur-rich wastes. Reactor experiments showed that biochar redirected about 26.6% of electrons from sulfide production to methane generation without suppressing sulfate-reducing bacteria. Life cycle assessment further highlighted the environmental benefits of this electron diversion for biogas desulfurization and bioenergy recovery, underscoring biochar role in enhancing AD efficiency and sustainability. In short biochar enhances energy storage, fuel cells, and waste-to-energy systems via its porosity, stability, and catalytic properties.

4.5. Carbon Sequestration and Climate Change Mitigation

The long-term stability of biochar carbon in soils positions it as an effective tool for carbon sequestration and climate change mitigation. One of the primary advantages of biochar in climate change mitigation is its ability to sequester carbon durably within soil environments. The global potential for biochar-mediated carbon sequestration is substantial. Current estimates suggest that widespread biochar application could offset 12% of global anthropogenic greenhouse gas emissions, equivalent to approximately 1.8 Gt CO₂ per year [103]. Wang Y. et al. [104] conducted decade-long field experiments and showed that applying biochar to calcareous farmland topsoil increased the soil organic carbon (SOC) by 33%. Subsoil DOC contents were increased by 162%, while a 5% increase in dissolved organic carbon (DOC) content was observed in the topsoil. A 24% increment in humic substances was observed in the topsoil and a 142% increase in molecular-weight water-extracted DOC in the subsoil. The application of biochar significantly increased the contents of SOC, DOC, and microbial biomass carbon (MBC) in the topsoil, as well as SOC and DOC contents in the subsoil. However, a slight decrease was observed in the MBC content in the subsoil. Biochar-amended soil significantly suppressed enzyme activity in the topsoil and decreased α diversity in both the topsoil and subsoil, while increasing the content of mineral-associated soil organic matter (MAOM) (Figure 14a). Ding X. et al. [105] reported an 11-year field experiment to evaluate biochar application rates of 0, 30, 60, and 90 Mg ha⁻¹ on native soil organic carbon (SOC) fractions and microbial activity in calcareous soil under a winter wheat–maize rotation. After 11 years, 39–51% of the applied biochar remained in the top 0–30 cm soil layer. Biochar increased the native SOC and recalcitrant SOC contents but reduced the proportion of native labile SOC pools I and II, dissolved organic carbon, and microbial biomass carbon. Higher biochar rates (60 and 90 Mg ha⁻¹) significantly increased the C₄-derived CO₂ contribution to soil basal respiration and enhanced native SOC mineralization, with effects confined to the 0–15 cm layer. Soil depth increased the labile SOC pools and reduced the metabolic quotient (qCO₂), indicating that the biochar rate and soil depth jointly regulate long-term SOC turnover and sequestration (Figure 14b). Guo F. et al. [106] investigated the effects of a 7-year biochar application on native soil organic carbon (n-SOC) dynamics in woodland, lawn, and greenhouse soils. After 7 years, 67.12%, 87.50%, and 88.13% of the applied biochar remained in woodland, lawn, and greenhouse soils, respectively. Biochar increased n-SOC by 2.07, 3.07, and 0.22 times in the three ecosystems, primarily through enhancement of the native humin content. Biochar increased large aggregate proportions and aggregate-associated total SOC in woodland and lawn soils, while only increasing aggregate SOC in greenhouse soil without altering aggregate distribution. Native easily oxidizable carbon and microbial biomass carbon increased across all soils; however, elevated microbial biomass in woodland and lawn soils accelerated hot-water-extractable organic carbon depletion and CO₂ emissions. Biochar reduced microbial respiration quotient (qCO₂) in woodland and greenhouse soils but had no effect on lawn soil, while the carbon use efficiency declined in lawn soil due to reduced fungal-to-bacterial ratios.

Salem I.B. et al. [107] evaluated the potential use of date palm leaf biochar for CO₂ capture and sequestration for climate change. They used different pyrolysis temperatures from 300 to 600 °C, with an increment of 100 °C at each step. The physicochemical characteristics of the synthesized biochar were examined through advanced techniques. According to the results, date palm biochar as a porous carbon-based material exhibited high CO₂ adsorption capacity through physisorption and chemisorption progressions. The CO₂ capture percentage increased with pyrolysis temperature, with 0.09 kg, 0.15 kg, 0.20 kg, and 0.25 kg CO₂/kg palm biochar synthesized at 300, 400, 500, and 600 °C, respectively (Figure 14c).

Biochar application reduces net greenhouse gas emissions from soil through multiple mechanisms, effectively enhancing the climate mitigation benefit beyond direct carbon sequestration. Biochar significantly reduces nitrous oxide (N₂O) emissions of greenhouse gas, which is 300 times more potent than CO₂, by 12–84% depending on application rate, soil type, and environmental conditions [108]. While biochar sequestration aims to stabilize atmospheric carbon captured by plants, the literature shows that it does not create a perfectly closed carbon loop. During pyrolysis, a large part of the biomass carbon is lost as CO₂, CO, and CH₄ [109]. Even though biochar carbon stays in the soil much longer than raw biomass (about 74% remains after 100 years), it still slowly breaks down over centuries [110,111]. Therefore, biochar is better described as a negative-emission technology whose real effectiveness depends on minimizing emissions during production and using low-carbon energy.

Table 2. Experimental applications of biochar in environmental and energy systems, showing key metrics, functional roles, and associated benefits; only experimental studies with quantified results are included, with references.

No.	Source & Preparation	Application	Metrics (Measured)	Significance of Study	Ref.
1	Reed straw, apple wood, and corn straw biochar	PMFC power enhancement	Peak power density of ~1608 mW m−2 in PMFC	Demonstrated substantial enhancement of electricity generation in plant microbial fuel cells	[112]
2	Bamboo biochar added to AcoD	Anaerobic co-digestion methane enhancement	Specific methane production ↑ 42.56% with biochar and CO2 addition	Boosts methane yield and microbial stability	[113]
3	Biochar at 10 g/L with grass silage + cattle slurry	Methane yield in two-stage AD	CH4 yield 253 L/kg VS (2-stage) and 218 L/kg VS (single-stage)	Improves biomethane production efficiency	[114]
4	Corn stover & varied biochar	Enhanced methane from multiple biochar	Maximum methane yield of 218.45 ± 9.55 L kg−1 VS, up to 86.14% higher than control	Significantly increases biogas energy output	[115]
5	Corn stover biochar (biochar dose 1.82 −3.06 g g−1 TS)	Enhanced biogas from AD	Methane content increased from ~67.5% to 81.3–87.3%; VS destruction improved by 14.9%	Enhances methane concentration and digestion stability	[116]
6	Corn stover biochar	Pilot-scale enhanced AD	Biogas production of 368.6 L kg−1 VS and methane yield of 230 L kg−1 VS (35–37% increase)	Demonstrates improved methane productivity at pilot scale	[117]
7	Corn stover + chicken manure biochar	Co-AD with urea pretreatment	Volumetric methane production increased by 32.8–96.4% with combined biochar and urea treatment	Enables high-loading anaerobic co-digestion with improved gas yield	[118]
8	Pepper straw biochar	DC-SOFC fuel	Peak power density of 217 mW cm−2, fuel utilization of 44.4%, and operational lifetime ~21 h	Confirms feasibility of biomass-derived biochar as a DC-SOFC fuel	[119]
9	Rice & corn straw biochar	DC-SOFC improved performance	Peak power density reaching up to 338 mW cm−2	Improves electrochemical performance of DC-SOFC systems	[120]
10	DC-SOFC comparative fuels	Wheat straw, corncob, and bagasse char	Power density ranging from 187 to 260 mW cm−2 at 800 °C	Validates agricultural biochar as alternative high-temperature fuel cell fuels	[121]
11	Fermentation residue	Biohydrogen catalyst	Cumulative hydrogen production of 570 mL using BC3	Enhances biohydrogen generation efficiency	[122]
12	Co-Fe-N doped biochar	Biohydrogen surge	Hydrogen production increased by 367% (151 to 589 mL)	Promotes hydrogen-producing metabolic pathways	[123]
13	Corn stover slurry	Co-digestion with membrane + biochar	Cumulative methane production of 137.14 mL g−1 VS, representing a 26.5% increase	Improves methane recovery from digestion slurry systems	[124]
14	Date palm leaf biochar (300–600 °C)	CO2 capture	CO2 adsorption capacity of ~0.017 g g−1 at 500 °C	Demonstrates potential for carbon dioxide capture and sequestration	[125]
15	Sewage biochar	Sewage SS biochar AD	Methane production increased by 10–31%	Improves anaerobic digestion efficiency of sewage sludge	[126]
16	Long-term AD methane gains	Algal & food waste biochar	Methane yield enhancement of up to 54% during long-term digestion	Supports sustained improvement in methane production	[127]
17	Biochar electrodes	Supercapacitors	Specific capacitance of approximately 114 F g−1	Enhances charge storage performance in energy storage devices	[128]
18	Algal biomass biochar electrode	Algal biochar MFC anode	Power density of 6.8 W m−3 and coulombic efficiency of 9.33%	Integrates electricity generation with wastewater treatment	[129]
19	Bamboo biochar SMFC	SMFC biochar boost	Internal resistance reduced by 29.8–57.5% and output voltage increased up to 3.2-fold	Enhances sediment microbial fuel-cell performance	[130]
20	Sorghum biochar	Sorghum stalk AD	Biogas production of ~22.8 mL g−1 VS compared with 11.7 mL g−1 VS in control	Demonstrates effective waste-to-energy conversion	[131]
21	Biochar-modified anode	MFC brewery wastewater	Maximum power density of 108.05 mW m−2	Enables simultaneous bioelectricity generation and organic matter removal	[132]
22	Biochar–rutile cathode	Biochar MFC with rutile	Power density of 10.44 mW m−2	Achieves combined electricity generation and heavy-metal removal	[133]

^↑ Denotes significant increase in measured metrics.

provides experimental applications of biochar in environmental and energy systems, highlighting the sources, target applications, measured performance metrics, and the main benefits observed. Only studies with quantified experimental results are included.

5. Conclusions

This review synthesizes a decade of advances (2017–2026) in biochar production and applications, highlighting that its performance can be strategically engineered through feedstock choice and pyrolysis conditions. Key findings indicate that agricultural residues yield oxygen-rich, functional material ideal for nutrient retention and soil amendment, while woody biomass produces stable, aromatic char suitable for long-term carbon sequestration and CO₂ capture. Manure- and sludge-derived chars offer high alkalinity and cation exchange capacity, which is beneficial for fertilization and wastewater treatment, though with trade-offs in stability.

Low-temperature products (300–400 °C) excel at heavy-metal complexation and cation exchange. Intermediate temperatures (400–600 °C) balance stability and reactivity for agricultural and wastewater applications. High-temperature chars (>600 °C) deliver superior aromaticity, surface area, and long-term recalcitrance for carbon sequestration, catalysis, and energy systems. Surface area, porosity, functional groups, ash content, and pH collectively govern sorption mechanisms and biomass degradation.

Despite significant progress, inherent trade-offs remain. High-temperature chars exhibit reduced surface functionality and lower CEC, limiting nutrient retention and certain metal complexations. Low-temperature products retain abundant functional groups and high CEC but decompose faster, constraining long-term sequestration potential. Excessive ash and alkalinity, particularly in manure- and sludge-derived materials, can be advantageous for acidic soil remediation but may induce salinity stress or operational issues in electrochemical systems. Activation and doping strategies enhance the performance but increase the cost, complexity, and scalability concerns. These findings underscore that no single approach is universally optimal, and performance is strongly application-specific.

Future research should prioritize application-specific design rather than generic production. Targeted studies are needed to optimize pH-dependent biomass degradation. Alkaline high-temperature chars could be engineered to accelerate lignocellulosic breakdown in anaerobic digestion or composting while minimizing nutrient loss. Moderate-temperature products with preserved functional groups may support controlled microbial and enzymatic activity for balanced decomposition and nutrient mineralization. Additional priorities include standardized, comparative experiments across feedstocks and pyrolysis conditions, long-term field trials evaluating carbon stability and microbial effects, techno-economic and life-cycle assessments of scalable production, and development of hybrid or co-engineered materials (e.g., with microbes or mineral amendments) to overcome single-temperature limitations. Addressing these gaps through controlled, application-oriented approaches is essential to translate laboratory advances into reliable, sustainable technologies for environmental remediation, energy production, and climate mitigation.