Adsorption Mechanisms in Bone Char for Water Treatment

Bone char is a carbonaceous adsorbent derived from the pyrolysis of animal bones, yielding a composite of graphitic carbon and calcium phosphates, predominantly hydroxyapatite. Its bimodal structure facilitates multiple adsorption pathways. Physisorption occurs across the high-surface-area carbon matrix via van der Waals forces, effectively capturing organic molecules and cationic dyes. Simultaneously, chemisorption and ion-exchange processes operate at hydroxyapatite sites, where calcium and phosphate moieties interact selectively with inorganic contaminants such as fluoride and heavy metals. Pore size distribution governs mass transfer: macropores and mesopores accelerate kinetics, while micropores enhance capacity. Surface functional groups introduced during charring further modulate affinity for specific solutes. Operational conditions—pH, temperature and contact time—critically influence ion speciation, surface charge and adsorption thermodynamics. Advanced spectroscopic and microscopic analyses have elucidated interfacial interactions, aiding the design of regeneration strategies. The versatility of bone char, its low cost and the valorisation of waste bones underscore its global relevance for decentralised defluoridation, industrial effluent polishing and emergency water provision, in line with circular economy principles.

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Technical terms

Bone char: Carbonaceous material obtained by pyrolysing animal bones, containing graphitic carbon and calcium phosphates.

Hydroxyapatite (HAP): A calcium phosphate mineral (Ca₅(PO₄)₃OH) that provides ion-exchange and chemisorption sites.

Physisorption: Reversible adsorption driven by weak van der Waals interactions on porous surfaces.

Chemisorption: Adsorption involving the formation of chemical bonds between adsorbate species and surface functional groups.

Ion exchange: A mechanism by which solution ions are swapped with adsorbent surface ions, enabling selective uptake.

Langmuir isotherm: A model describing monolayer adsorption onto a homogeneous surface with a finite number of identical sites.

Freundlich isotherm: An empirical model representing adsorption on heterogeneous surfaces with variable affinities.

References

1. Preparation, characterisation and applications of bone char, a food waste-derived sustainable material: A review. Journal of Environmental Management (2023).
2. Evaluation of Fluoride Adsorption Mechanism and Capacity of Different Types of Bone Char. International Journal of Environmental Research and Public Health (2021).
3. The adsorption study of Royal Blue Tiafix and Black Tiassolan dyes using bone char as adsorbent. Adsorption Science & Technology (2018).