Bovine Bone-Derived Mesoporous Hydroxyapatite as an Inorganic Pigment for Energy-Efficient Radiative Cooling Paints

Animal bone biowaste disposal in landfills creates environmental and health hazards. By recycling this hazardous waste for use in energy management applications, we can protect the environment from the spread of diseases and climate change. This study proposes a new technique to convert waste into inorganic pigments for preparing radiative cooling paint to aid in decarbonization. In this method, hydroxyapatite (HAp) is produced using a chemical-treated thermal decomposition process at various calcination temperatures ranging from 650 °C–850 °C. The structural, thermal, and optical properties of the prepared samples are analyzed using XRD, SEM, FE-SEM, MIP, TEM, BET, TGA, UV–Vis-NIR, and FT-IR. The chosen synthesis route produces ultrawhite organic-free HAp, and the structural characterization reveals that HAp3 has a polycrystalline nature with an average crystallite size of 41.78 nm and a high surface area like commercial HAp. Its mesoporous structure and varied particle morphologies enhance light scattering. HAp3's wide bandgap minimizes sunlight absorption, and phosphate ion vibrations improve emission, reducing solar absorbance from 42.22% to 7%. Applying HAp3 to asbestos cement sheets resulted in an 8.7 °C temperature reduction compared to untreated roofs, demonstrating the potential of recycled bone biowaste pigments for climate change mitigation.