Global applications of biochar in sustainable cities of the future: a perspective

Funmi M. Alayaki; Pouria Hajikarimi; Naira Meky; Suliman Rashid; Elham H. Fini; Funmi M. Alayaki; Pouria Hajikarimi; Naira Meky; Suliman Rashid; Elham H. Fini

doi:10.48130/bchax-0025-0009

Production requirement	Thermochemical conversion technologies of biomass
Production requirement	Combustion	Gasification	Fast pyrolysis	Torrefaction (slow pyrolysis)	Liquefaction
Pre-treatment of biomass	Drying and appropriate particle sizes	Drying and appropriate particle sizes	Drying and appropriate particle sizes	Wet processing under high pressure can be used without first drying	Not required
Moisture content in biomass	< 50%	< 30%	< 10%	Subjective	> 90%
Temperature range (°C)	800−1,000	800−1,200	400−500, low residence time and rapid cooling of the vapour (< 1 h)	200−350 and long reaction time	150−450
Pressure	Not required	Not required	Not required	Not required	Pressurized solvent 1−240 bar
Oxygen	Require	Partial oxidation or air atmosphere	Not required	Not required	Not required
Energy produced	Direct use as heat	Stored as chemical energy	Stored as chemical energy	Product (low grade charcoal) can be densified into pellets or briquettes to obtain higher energy density	Liquid product

Table 1.

Production requirements for some thermochemical conversion technologies of biomass

Biochar feedstock/modification	Target aromatic VOS (s)	Adsorption capacity (mg/g)	Key mechanism/finding	Ref.
Wheat straw, hardwood; NaOH-benzoic acid modification	Toluene p-xylene	~2× increase vs unmodified	Increased surface area, oxygen functional groups (hydroxyl, carboxyl) enhance adsorption	[11]
biochars from various feedstocks, pyrolyzed at 300–600 °C	Toluene, acetone, cyclohexane	5.58–91.2	Surface area and noncarbonized organic matter are key; lower pyrolysis temperature resulted in higher capacity	[35,38]
Wheat straw, corn straw, bagasse	Toluene, p-xylene, hexane, acetone	51–110 (single), 50–109 (mixed)	Bagasse biochar had highest capacity; surface area and pore volume critical	[49]
Algae	Non-polar aromatics, e.g., benzene derivatives	Emission reduction: 76% (iron-rich) vs 59% (low-iron)	Fe content and N–Fe bonds boost adsorption and catalytic degradation	[50]
Rapeseed cake, walnut shells; H₂SO₄ and KOH activation	Toluene, acetone	Up to 166.7 (H₂SO₄-activated)	Acid activation increases surface area and pore volume, enhancing adsorption	[51]
Engineered biochar (hydrophobic, porous)	Benzene, toluene	Benzene: 136.6, toluene: 94.6	Hydrophobicity and porosity increase selectivity for aromatics, reduce water vapor uptake	[52]
Straw–sludge blend, CO₂/H₂O activation	Toluene	5.1× higher than activated biochar, 3.2× than commercial	Physical activation with CO₂/H₂O is cost-effective and environmentally friendly	[53]

Table 2.

Summary of removal mechanisms of aromatic VOCs by various types of biochar

Biochar feedstock/modification	Target aromatic VOS (s)	Adsorption capacity (mg/g)	Key mechanism/finding	Ref.
Rice husk, straw, coffee grounds	Alkanes, alkenes (from asphalt)	Rice husk biochar showed highest VOC reduction	Feedstock type and morphology critical for adsorption performance	[55]
Pomegranate peel biochar	Hexane	100% reduction within 480 minutes	Hydrophobic interactions and Van der Waals forces drive efficient hexane capture	[56]
Iron-rich biochar	Heptane, cycloheptane	76% emission reduction (iron-rich) vs. 59% (low-iron)	Iron content and N–Fe bonds enhance adsorption and catalytic degradation	[50]
Poultry litter, swine manure, oak, coconut shell biochars	Volatile fatty acids, reduced sulfur compounds (e.g., acetic acid, DMDS, DMTS)	Oak biochar (500 °C) showed high sorption capacity; plant-biomass biochars better than manure-based	Sorption capacity varies with feedstock; plant-based biochars better for sulfur compounds; potential soil amendment reuse	[57]
Wood shaving, chicken litter biochars (activated and non-activated)	Gaseous ammonia (NH₃)	Adsorption capacity: 0.15–5.09 mg N/g	Phosphoric acid activation greatly increases NH₃ adsorption; surface acidic oxygen groups important	[58]
Pine needle biochars pyrolyzed at 100–700 °C	Nonpolar non-aromatic VOCs (e.g., naphthalene)	Sorption capacity varies with pyrolysis temperature	Sorption mechanism shifts from partitioning to adsorption with increasing pyrolysis temperature	[29]

Table 3.

Summary of removal mechanisms of non-aromatic VOCs by various types of biochar

Application	Biochar functionality	Key findings/advantages	Ref.
Heavy metal removal	Adsorption of heavy metals (Pb²⁺, Cd²⁺, Cu²⁺, Zn²⁺, Ni²⁺) through biochar's porous structure and functional groups	- High affinity for Pb²⁺ over Cd²⁺ - Maintains > 70% removal efficiency after multiple cycles - More cost-effective than activated carbon	[88,89]
Dye removal	Adsorption of complex dye molecules utilizing mesoporous and nanostructured biochar with high surface area.	- Removal efficiencies up to 95% (e.g., Rhodamine B) - Effective against various dye types ( direct, acid, reactive) - Reusability over multiple cycles	[90,91]
Nutrient (N & P) removal	Adsorption of nitrogen (ammonium, nitrate) and phosphorus (phosphate) via ion exchange and surface complexation, often enhanced by metal loading.	- Iron-loaded biochar removes > 85% TP and 60% TN with minimal desorption - Effective removal of organic and inorganic phosphorus from industrial wastewater	[92]
Organic contaminants removal	Adsorption and catalysis of recalcitrant organic pollutants (pesticides, antibiotics, herbicides) through surface functional groups and porous matrices.	- Up to 70% removal rate of sulfonamide antibiotics - Maximum adsorption capacity ~ 80 mg/g for glyphosate - Facilitates degradation via catalyzed redox reactions	[85,93,94]
Emerging contaminants (PFAS)	Functionalized biochar with enhanced surface chemistry for adsorption of PFAS through electrostatic, hydrophobic interactions and ion exchange.	- Functionalized biochars improve PFAS removal efficiency - Long-chain PFAS adsorbed more effectively than short-chain - Optimal biochar dosage required	[95]
Pathogen removal	Adsorption and entrapment of pathogens within biochar's porous structure and antimicrobial surface functionalities.	- Effective removal of bacterial and viral pathogens - Enhances microbial activity beneficial for pollutant biodegradation	[87]
Additive in anaerobic digestion	Enhancement of anaerobic digestion process by improving microbial activity, accelerating methane production, and speeding up degradation reactions.	- Reduction of lag time by up to 30% - Increase methane yield by 27%–42% - Enhances COD removal by 50%	[96−98]

Table 4.

A summary of the diverse applications of biochar in water and wastewater management