The international shellfish farming industry has a growing problem with respect to sustainability: the shells, a by-product, are currently being mostly wasted to landfill. Landfill space however is becoming increasingly expensive and hard to obtain. Instead, this calcium-rich resource could be used to produce a value-added product. As such, this paper presents the initial work towards using this shellfish waste to produce hydroxyapatite.

Powdered mussel shell was heat treated to form lime. Shell particle size, pyrolysis time, temperature and shell concentration were varied to determine their effect on lime formation and its reactivity with phosphate to form hydroxyapatite. By adding the shell-derived lime directly to the phosphate, the best performing shells (particle size: 53 - 106 um, pyrolysed for 1 hour at 750°C) reacted 90% of the phosphate to a solid with just 196 mg L^-1 of shell. Analysis by environmental scanning electron microscope (ESEM) combined with wet chemical analysis of the phosphate content in the liquid, the resulting precipitate and on the remaining shells after reaction, showed that for pyrolysed shells the main mechanisms of phosphate removal are homogeneous nucleation of hydroxyapatite to form a suspended precipitate, as well as adsorption and heterogeneous precipitation on the surface of the remaining calcite shell particles. Therefore to obtain pure hydroxyapatite, further experiments dissolving the lime and removing the remaining shells were conducted. The precipitate obtained demonstrated that this process is feasible. The mass of precipitate increased with both increasing temperature and reaction time, allowing the process to be further optimised.