Indonesia produced 67,201,200 kg of eggshell waste as the world’s eighth-largest egg-producing nation in 2017. Most of it goes to landfill, where it decomposes into ammonia, hydrogen sulfide and amines, and attracts what that attracts. Converting that stream into a biomaterial is a genuinely good idea, and it is the idea this entire industry rests on.
It also means the feedstock is an agricultural product, with an agricultural product’s variability. So the useful question is not whether eggshell makes good hydroxyapatite. It is what else comes along.
What a shell is made of
El Milla, Hartami, Priyanto and Adrian, working at Universitas Brawijaya in Malang, published exactly that measurement in the Malaysian Journal of Medicine and Health Sciences in April 2023.
They start with the shell itself. Calcium carbonate makes up 94–97% of an eggshell’s chemical components, with 3–4.5% organic content. Alongside it sit small quantities of other things: MgO at 0.83%, SO₃ at 0.66%, SrO at 0.43%, plus P₂O₅, Al₂O₃, K₂O, SiO₂ and Cl₂O₃.
Their material was made by the hydrothermal route from Golden Comet chicken eggshells collected in Serang, Banten — milled to 100 mesh, calcined at 1000 °C for five hours to convert the carbonate to calcium oxide, then reacted with diammonium hydrogen phosphate at a Ca/P mole ratio of 1.67 and held at 230 °C for 48 hours. Particle size analysis put the average grain size at 243.4 ± 5 nm.
98.5, and the remainder
The X-ray diffraction result is a good one. The synthesised material came out at 98.5% hydroxyapatite.
The remaining 1.5% was silica — and the paper does not leave its origin ambiguous. The impurity, it says, “came from the egg shell which was used as the starting material for the synthesis.”
That is the sentence worth keeping. The contaminant was not introduced by poor handling or a dirty vessel. It was in the feedstock, it survived 1000 °C and 48 hours of hydrothermal synthesis, and it arrived in the product. There were also peaks in the diffractogram the authors could not assign — “not compounds of the elements Ca, Si, O, H, and P” — which they marked and reported rather than quietly smoothing away.
Their own conclusion notes the material is a good candidate “but other strategy needed to eliminate the impurities especially for tissue regeneration material.”
Why the trace table exists
Set this beside ISO 13779-1, which caps arsenic at 3 mg/kg, cadmium at 5, mercury at 5 and lead at 30, with a total ceiling of 50 mg/kg for metals with adverse biological reactions.
Those limits can look like bureaucratic furniture until you have read a paper that traces a product impurity directly back to the raw material it was made from. A biogenic feedstock inherits its source’s composition — the beneficial trace elements everyone advertises, and the ones nobody puts on a label, by the same mechanism. Magnesium and strontium are sold as advantages. Silica arrived by exactly the same route and is sold as nothing at all.
None of which argues against eggshell as a source. It argues against treating “naturally derived” as though it settled the purity question, when it is the reason the question has to be asked.
Chickens are fed differently in different places, and their shells reflect it. Which makes the composition of the feedstock a supplier variable like any other — one that a serious producer measures per lot, and an unserious one describes in a marketing sentence.
Sources
El Milla L, Hartami E, Priyanto R, Adrian AGW. “Synthesis and Characterization of Eggshell Based Hydroxyapatite Nanoparticles: X-ray Diffraction and Spectroscopy Analysis.” Malaysian Journal of Medicine and Health Sciences, 19(SUPP5):15–18, April 2023 (open access) — medic.upm.edu.my. Trace-element limits from ISO 13779-1:2008(E), Table 1 — iso.org/standard/43826.html.