Biochar is fabricated from lignocellulosic biomass using thermo-chemical approaches such as pyrolysis (slow and fast), torrefaction, carbonization (hydrothermal or flash), and gasification (Amusat et al., 2021). The feedstock material is thermochemically or pyrolytically decomposed at 350–700°C in vacuum (<1% O₂) for the generation of BC. Slow pyrolysis is an eco-friendly process commonly employed for BC fabrication and results in high yield of bio-oil and 35% yield of dry mass (Tomczyk et al., 2020). Zhang et al. (2017) used slow pyrolytically produced BC for soil remediation and sorption of different pollutants from wastewater. For biofuel production, fast pyrolysis is favored over other processes; gasification is primarily employed for the synthesis of syngas, which subsequently produces heat and energy. Additionally, lignocellulosic material caused higher BC yield than municipal solid waste (Ashiq et al., 2019). To expedite the nano-BC production the employment of BC produced by traditional thermochemical methods is advised, while optimizing the quality of biomass materials through transforming them to nano-particles. The traditional methods produce BC with different yields and elemental constitution (C, H, O). Biomass normally becomes more carbonized as treatment intensity increases, which corresponds to an elevation in C composition but a reduction in O and H composition. Additionally, BC is modified physically and chemically for a variety of purposes to enhance its functionality. Steam coating, chemical oxidation, acidic/basic treatment, CO₂ activation, saturation with native and artificial nano-materials is used for chemically modifying BC (Song et al., 2022).

Nano-BC inherently forms while synthesizing macro-BC, however its output is limited (<2.0% from peanut shell-derived BC) (Liu et al., 2018). It is thus necessary to undergo a size reduction process in order to enhance the amount of nano-BC (Table 1). The production of nanomaterials can be performed by top-down or bottom-up processes. In the top-down process, the size of the macro BC is minimized to nanoscale; whereas in the bottom-up method, the nanomaterial is amassed from the atomic level. Top–down methods such as grinding, cutting, centrifugation and etching are used for the fabrication of nano-BC in an economical manner. Lonappan et al. (2016), Dong et al. (2018), Lian et al. (2020), and Ramanayaka et al. (2020a) have employed grinders for reducing the size of macro-BC to nanoscale. The bottom up method includes disintegration by ball milling or sonication and carbonization. Ball milling enables fabrication of nano-BC with improved properties without destroying its crystal structure (Amusat et al., 2021). Ball-milling has received great attention because of its low cost and energy demand during manufacturing, eco-friendly nature and wide range of application. The ball milling method disintegrates bulk-BC into nanoscale by the colliding it between metallic balls. The desired particle size may be attained by regulating the aggregation and modifying the ball sample ratio and milling time. There are two ways to ball mill BC at the nanoscale: wet and dry techniques. The wet approach is more preferred due to synthesis of nano-BC with superior dispersivity, higher surface functionality, eco-friendly and less labor intensive approach (Yuan et al., 2020). Ball milling method effectively tailors nano-BC characteristics by enhancing surface area, decreasing material size, improving surface oxygen functionality, and increasing sorption and photocatalytic efficacy (Lyu et al., 2017, 2018; Naghdi et al., 2017; Wang et al., 2018). Nano-BC with particle size smaller than 100 nm was fabricated within 30 h using planetary ball milling method (Richard et al., 2016). Naghdi et al. (2019) suggested a pre-treatment of BC at 80°C for 24 h before conversion to nano-BC using planetary ball milling within 100 min. The pre-conditioning subdued the agglomeration of nano-particles and reduced the size of BC from 212 to 60 nm. Double-disc milling is also an alternative method for nano-BC fabrication; however it demands high operational costs. Among the different ball mill methods, vibrating disc milling produces greater quantities of nano-BC with consistent size and shape due to attrition and shear stress (Karinkanta et al., 2018). Several studies have reported fabrication of nano-BC in a controlled environment using process parameters such as milling period of 120–1,200 min, number of balls from 25 to 800, ball weight from 0.5 to 100 g, and ball size from 3/4 to 15 mm. Ma et al. (2022) optimized the process parameters for synthesis of ball-milled nano-BC by regulating grinding time, rotating speed, and ball-to-powder mass ratio. The BC mixtures must be subsequently dispersed in different solvents post milling to improve particle distribution before separation (Song et al., 2022). The pre-treatment at 80°C however enables the reduction in size aggregation, but is a high-cost method thus restricting its scale-up. Iron oxides can be also added into BC for suppressing the agglomeration of particles and enhancing their distribution (Li et al., 2020a). Ball milling is a high atom economy method and generates nano-scale biodegradable products using renewable sources by limiting the usage of hazardous chemical-assisted procedures.

Among physical methods, sonication is an efficient method for production of nano-BC by employing high-energy ultrasonic radiations to disintegrate BC in suspension. The microporous region in BC increases due to shock waves resulting in opening of clogged pores and exfoliating the carbon structure. The small exfoliated particles then adhere to the surface or embed in the pores of BC resulting in nano-BC production (Liu et al., 2018). The uniformity in nano-BC surface and the development of porosity without obstruction are two prime benefits of sonication (Yang Y. et al., 2020). Few investigations also reported the generation of nano-BC from waste lignin carbonization as a post or pre-treatment with milling for enhancing the surface features and size of nano-BC with subsequent removal of impregnating salts (Jiang et al., 2020; Makshut et al., 2020). Guo et al. (2020) employed hydrothermal reaction for fabricating nano-BC from agricultural waste biomass. Soybean straw and animal wastes were employed as feedstock and digested with acids in a high-pressure hydrothermal reactor. Furthermore, multiple rounds of centrifugation are also employed for separating highly dispersed nano-BC particles (Ullmann et al., 2017). Different feedstock material and pyrolytic conditions, centrifugation period (2–30 min) and rotational speed (3,500–1,000 rpm) were used to prepare nano-BC (Anupama and Khare, 2021).

The intrinsic characters of nano-BC can be readily modified thus providing a platform for the easy modification for wide applicability in different sectors. Surface fictionalization using amination, sulfonation and oxidation improves the performance of BC-based nanomaterials (Nath et al., 2019). It has been observed that employment of different combination of pure and acid mixtures (H₂SO₄, HNO₃, and HCl) for surface functionalization increased carboxylic group formation with concurrent laccase adsorption (Naghdi et al., 2017). Similarly, Fe₃O₄ engineered nano-BC has a greater surface area and adsorption site owing to mesoporous structure (Nath et al., 2019). Cellulosic nanocrystal derived nano-BC was modified with ZnO and it displayed greater active sites and functioned as potent photo-catalysts for phenol removal (Zhang et al., 2021). Nano-BC obtained from artichoke leaves was base-modified with NaOH and employed for removal of metformin hydrochloride. The results revealed the existence of COOH, OH, and C=C groups and higher elimination rates of metformin by modified nano-BC in comparison with pristine nano-BC (Mahmoud et al., 2020). Ethylenediamine functionalized nano-BC was employed as an effective nano-sorbent for removing prednisolone and Cr(VI) (Mahmoud et al., 2022). The significance of magnetic nano-BC for treating tetracycline and Hg(II) polluted wastewater was assessed by Li et al. (2019b). The modified nano-BC exhibited high removal rates (>99%) for both tetracycline and Hg(II) (Li et al., 2019b). The employment of pristine BC imposes some limitations on the adsorption efficacy for various contaminants due to low surface functionality and pore size. The surface modification of BC by different approaches improves the surface area, furnishes additional surface functional groups and adsorption sites. Thus, functionalized BC is a promising potential substitute for treating wide range of contaminants.

Nano-BC has demonstrated exceptional adsorption capability of hazardous organic and inorganic compounds, personal care products, pharmaceutically active compounds, insecticides, and heavy metals from various environmental matrices (Ma et al., 2022; Jiang et al., 2023; Mahmoud et al., 2023). The excellent sorption ability of nano-BC is due to the generation of a stable colloidal solution, greater surface areas, porosity, and surface charge. Chemical and physical adsorption, precipitation, and ion-exchange are the three primary process described for the adsorption of inorganic pollutants on nano-BC (Amusat et al., 2021). The carboxyl, phenol and hydroxyl groups present on nano-BC surface assist in chemi-sorption of contaminants by exchanging anionic ions with cationic contaminants. Physi-sorption occurs due to electrostatic and Van der Waals interactions among the freely mobile electrons of surface aromatic functional groups of the derived nano-BC, ultimately resulting in non-covalent attraction with C=C bonds (Amusat et al., 2021). Precipitation is also considered as one of the primary processes of sorption of inorganic contaminants. It generally involves heavy metal ion precipitation onto the nano-BC surface either in solid form or in the solvent during the adsorption process. The adsorption of organic contaminants by nano-BC consists of different sorption mechanisms including; electrostatic and hydrophobic interaction, ion exchange and pore–filling (Rajput et al., 2022). Moreover physical sorption is regarded as an initial removal mechanism indicating that nano-BC may transport and subsequently desorb the toxic contaminants from aquatic environment. High temperature pyrolysis enhances the specific surface area and void structure richness of nano-BC and reduces hydrophilic surface functional groups and thus physic-sorption is the prominent mechanism for adsorption. However, the surface area of low-temperature pyrolyzed nano-BC is comparatively low and hydrophilic surface functional groups are high, suggesting chemi-sorption to be the dominant mechanism of adsorption (Jiang et al., 2023).

Elbehiry et al. (2022) studied the mono-and multi-sorption of metals (Cd, Cr, and Ni) on water hyacinths and black tea derived BC and nano-BC as a potent, economical and environmentally acceptable absorbents. The nano-BC eliminated >98.8% of Cr and Cd in mono-and competitive systems and the Freundlich isotherm model fitted most appropriately in the sorption kinetics (Elbehiry et al., 2022). The elimination of Cd(II) from an contaminated systems by nano-BC embedded in Ca-alginate beads and fabricated using ball-miller were reported (Wang et al., 2018). The improved surface functionality (oxygen-consisting groups) worked as effective sorption sites, promoting Cd(II)-calcium(II) ion exchange. The pH-dependent variations in Cd(II) sorption revealed the significance of oxygen-consisting surface groups (carboxylic, lactonic, and hydroxyl). The adsorption potential of nano-BC is determined by the surface area, humic acid, functional groups, and graphitic nature. Magnetic nano-BCs (nano zero-valent iron, iron sulfide, and iron oxide BC) display better functionality and magnetic properties that permit nano-BC recovery for recurrent usage. Improved chemical reduction, chemical precipitation, electrostatic interaction, surface complexation, ion exchange and radical activation due to synergistic effect of iron and nano-BC composites, better removal rate for wide range of pollutants are reported (Lyu et al., 2020a; Li et al., 2020b). Sisay et al. (2023) reported that Mg/Zr modified nano-BC derived from spent coffee grounds is an efficient sorbent for phosphate recovery and phosphorous release fertilizer. Furthermore, thiol-modified ball-milled BC demonstrated an improved Hg(II) elimination efficiency, with a sorption rate of 320.1 mg g⁻¹, as compared to unmilled BC (105.7 mg g⁻¹) (Lyu et al., 2020b). The amino-functional silica-coated magnetic nano-BC derived from Cynodon dactylon exhibited improved adsorption rates of 220.4 and 185.4 mg g⁻¹ for Cu and Pb, respectively. The nano-composites also demonstrated a 15-fold reuse capability and highest elimination rates for Cu and Pb (Dhanya et al., 2022). The efficacy of ethylenediamine modified nano-BC in the elimination of Cr (VI) and prednisolone was investigated (Mahmoud et al., 2022). Electrostatic, hydrophobic and π-π interaction, ion exchange and complexation are the reported processes for Cr (VI) and prednisolone sorption onto the modified nano-BC. Similarly electrostatic interaction was involved in the elimination of anionic inorganic contaminants from aqueous systems by CuO modified nano-BC. CuO provides cationic sorption sites on surface of nano-BC for the contaminant binding and its subsequent elimination (Wei et al., 2020).

Bulk-BC is the most commonly employed sorbent for treating a wide range of toxic contaminants (Yang F. et al., 2020). Nonetheless, nano-BC has an advantage above macro-BC due to larger specific surface area, higher negative Zeta-potential and greater surface functionality (Lian et al., 2018). Ball-milled BC displayed adsorption capabilities of 100.3 and 57.9 mg g⁻¹ for removing antibiotics sulfamethoxazole and sulfapyridine, respectively (Huang et al., 2020). According to Shen et al. (2020), any changes in mechanical, physico-chemical, and morphological characters of nano-BC may improve its adsorption efficacy. Luong et al. (2020) described that the change in pH from acid to alkaline using a detergent (tween 80) can increase the sorption capacities of pinewood nano-BC by up to 63%. Xiao et al. (2020) reported that goethite modified peanut shell nano-BC exhibited intercalated hetero-structures and improved hetero-aggregation, which resulted in better adsorption rates. Few reports have employed nano-BC for efficiently removing oxytetracycline from aqueous systems (Li et al., 2020b; Ramanayaka et al., 2020b). The larger surface areas and higher oxygenic groups on nano-BC surface enabled remarkable removal of trichloroethylene with a degradation rate of 99.4% within 5 min, where nZVI-enhanced SO4• synthesis improved the degradation rate (Yan et al., 2015). The removal rate for dimethyl phthalate, diethyl phthalate, and dibutyl phthalate by BC-graphene nanosheets was substantially greater than that of bulk-BC. The aromatic groups on BC graphene nanosheets displayed π–π EDA linkages with the aromatic ring of dimethyl phthalate while hydrophobic groups are involved in dibutyl phthalate binding (Abdul et al., 2017).

The sorption of Cu(II), tylosin and sulfamethoxazole on nano-hydroxyapatite modified BC occurred due to electrostatic and π–π interaction, and hydrogen bonds (Li et al., 2020b). The occurrence of tylosin and/or sulfamethoxazole increased adsorption of Cu(II) significantly (Li et al., 2020b). Nano-BC obtained from hickory wood with specific surface area of 305 m² g⁻¹ efficiently adsorbed various compounds (acetone, cyclohexane, chloroform, ethanol, and toluene) with an adsorption rate in the range of 23.4–103.4 mg/g (Xiang et al., 2020). The volatile organic compounds easily diffuse through the pores of nano-BC to reach the interior during sorption, reaching equilibrium after about 1 h. The physico-chemical, morphological and mechanical characters of nano-BC such as pore size, specific surface area, BC composition and contaminant characters are the most important factors in surface sorption (Xiang et al., 2020). The volatile polar organic compounds (acetone, ethanol, and chloroform) are sorbed onto nano-BC by dipole–dipole interaction and hydrogen bonds. The sorption of weakly polar volatile organic compounds exhibited more heterogeneity than the sorption of polar compounds. Furthermore, due to stronger intermolecular forces, organic contaminants with high boiling points (ethanol, cyclohexane, and toluene) were efficiently sorbed by nano-BC. Thus, the sorption of volatile contaminants by ball-milled nano-BC is a highly potent alternative for removal of air pollutants and warranting detailed research into the mechanisms and critical factors (Anupama and Khare, 2021).

Nano-BC can be utilized as an enzyme/microbe/biocatalyst carrier material for achieving continued breakdown of contaminants due to its high mobility and tunable surface chemistry (Table 4). Enzymatic catalysis is a sustainable process for degradation of pollutants and thus laccases immobilized nano-BC is being extensively employed for biodegradation of different contaminants. Oxygen functionalized nano-BC supported Lacasse was utilized for treating carbamazepine contamination (Naghdi et al., 2017). The acidic treatment introduced hydroxyl groups, with its subsequent oxidation to carboxyl groups. The degradation rates of >80% for carbamazepine with recyclability for 3 cycles at a comparative removal efficacy were reported (Naghdi et al., 2017). The immobilization process can be improved by addition of cross-linking compounds such as carbodiimide hydrochloride or glutaraldehyde (Naghdi et al., 2018). Naghdi et al. (2019) recently described a hydrogel technique for encapsulating laccase on nano-BC and chitosan. They reported that adding laccase onto nano-BC significantly improved its thermostability (4–70°C) during storage. Lonappan et al. (2018) used laccase immobilized nano-BC to completely remove (100%) diclofenac (2.5 mg L⁻¹) in 2 h, which was faster than laccase immobilized carbon nanotube (6 h), and 40% of the efficacy was retained post 5 cycles. More recently, laccase immobilized onto magnetic nano-BC was fabricated and employed for elimination of bisphenol A from aqueous environment (Zhang et al., 2020). The complete elimination of bisphenol A (25 mg L⁻¹) was reported within 75 min and 85% efficacy of the composite remained after 7 cycles. Cold-active toluene/o-xylene monooxygenase and catechol 1,2-dioxygenase were immobilized onto micro/nano-BC or chitosan and employed for petroleum hydrocarbon degradation (Miri et al., 2021). The results suggested that immobilization improved the storage stability of the enzymes (>50% recyclability after 1 month at 4°C) and degradative ability (>80% degradation of BTEX).

In recent times nano-BC supported photocatalysts have been fabricated through different methods to photo-catalytically breakdown water pollutants (phenolics, dyes, pharmaceutically active components, antibiotics). The photocatalysis is dependent on the methods of nano-BC synthesis. BC is an excellent support for photocatalysts owing to its tunable functional groups, chemo-stability, and electrical conductance. BC as a photocatalytic support reduces e−/h + recombination and thus displays increased catalytic efficiency (Ahmaruzzaman, 2021). Recently, the nano-BC/ZnO photocatalyst derived from carbon/ZnO nanocomposite were employed for photocatalytic breakdown of phenol (Zhang et al., 2020). In-situ precipitation and carbonization were used to create the photocatalyst, with carbon nano-composites serving as both the template and the carbon source. These composites demonstrated remarkable stability and durability with photodegradation rate of 95% suggesting that the integration of carbon nano-composites could efficiently reduce the photo-corrosion of ZnO (Zhang et al., 2020). The BC caused a reduction in the band gap of ZnO through continuous electron/hole separation and transport, increasing phenol degradation rates. A unique core-shell P-laden BC/ZnO/g-C3N4 nano-composite was recently reported as an excellent catalyst for atrazine breakdown and a potential regulated-release nanofertilizer for enhancing P utilization rates (An et al., 2022). The results revealed the formation of a Z-shaped heterojunction between ZnO and g-C₃N₄ in Pbi-ZnO-g-C₃N₄. The authors suggested that BC functions as an electron-transfer agent promoting the disjunction of electron–hole pairs. Highest atrazine photodegradation rates of 85.3% within 260 min were reported (An et al., 2022).

The electrochemical characteristics of nano-BC have recently received attention for its potential application as an alternative to carbon electrodes. The high adsorption capacity of nano-BC enables selective entrapment of chemicals to improve their concentration on electrode surface thus increasing the sensitiveness of electrochemical biosensors for detection (Table 4). The water-dispersible nature of nano-BC allows its use in film-forming methods for creating film electrodes with potent electrochemical applications (Plácido et al., 2019). In supercapacitors also, the electrode materials have recently been substituted with nano-BC due to its both meso and microporous structures having higher specific surface area resulting in improved performance. Biochar is also being used to replace cathode materials in batteries. Water dispersible nano-BC has been successfully used as an electrode material for Pb(II) and Cd(II) voltammetric sensors (Liu et al., 2016; Li et al., 2017). Li et al. (2017) investigated the impact of loading a hybrid of both bulk and nano-BC on the electrodes of a voltammetric sensor for detection of Pb²⁺, and increased sensitivity and electric current (3.24–4.0 and 4.5 μA) were reported. Moreover, fluorescence assay confirmed that BC releases dissolved organic matter containing fluorescent humic compounds (Hernandez-Soriano et al., 2016). The fluorescent characters of nano-BC have also been exploited for development of fluorescent detectors for metals (Plácido et al., 2019). Nano-BC fabricated from sorghum and rice straw and dairy manure was utilized as a probe for heavy metal detection and the results revealed a 100, 66, 66, and 33% accuracy for Pb²⁺, Ni²⁺, Cu²⁺, and Hg²⁺ detection, respectively, (Plácido et al., 2019). This was the initial report to suggest the application of nano-BC quenching data as a simple and accurate method for detection of toxic metal ions.

Furthermore, functionalized magnetic baggase nano-BC was fabricated by combining carboxyl groups and enzymes for bisphenol A monitoring in aqueous systems, demonstrating high sensitiveness as well as excellent electrochemical activity. Dong et al. (2018) studied the possibility of loading nano-BC on glass carbon electrodes for 17-estradiol monitoring and the electric current increased from 0 to 1.5 μA at the 17-estradiol amount of 3 M. The transfer resistance was lowered from 495 to 325 Ω on loading electrodes with nano-BC fabricated at 800°C in comparison with pristine BC resulting in increased electrode conductance. Ball-milled BC modified carbon electrodes displayed remarkable electrochemical characters and electrocatalysis as indicated by conductance, peak-to-peak disjunction, resistivity, and charge transfer resistance (Lyu et al., 2019). He et al. (2020) employed tyrosinase immobilized magnetic nano-BC for detection of bisphenol A. The developed electrochemical biosensor demonstrated a monitoring range of 2.78 nM with a linear range of 0.01–1.01 M, and the sensitivity remained consistent after 8 cycles without signal reduction. The use of nano-BC in electrochemistry can be expanded into new fields such as biomass electrocatalysis, fuel cells, and CO₂ reduction. A novel electrochemical biosensor for detection of lead and cadmium was synthesized by fabricating a high conductance and contaminant specific electrode. Ball milled BC was employed as the conductive material having large conductance, oxygen rich functionality and pores of ion-imprinted polymer functioned as target interacting sites (Mao et al., 2022). The ion imprinted bulk-BC electrode was created by in situ electro-polymerization of L-Cysteine and template metal ions on glassy carbon-modified bulk-BC, followed by template removal. The electrode detected very low concentrations of lead and cadmium using anodic dissolved differential pulse voltammetry. The monitoring range of 5.86 fM and 0.883 aM, and linear ranges of 25 fM ~ 1 μM and 0.1 fM ~ 1 μM, respectively were reported. The electrodes displayed no interaction with other ionic and organic molecules, and could be recycled for 7 cycles without losing detection sensitivity (Mao et al., 2022). Ferlazzo et al. (2023) described an electrochemical biosensor fabricated by nano-BC for the detecting nitrites and sulfites in contaminated water bodies. The nano-BC was placed on a commercial screen-printed carbon electrode (SPCE). The fabricated sensor outperformed the normal SPCE sensor in terms of detection limits and electrochemical oxidization of sulfites and nitrites in water (Ferlazzo et al., 2023).