Bio-Derived Dual-Temperature Antifouling Coating Enabled by Oyster-Shell Valorization

Oyster-shell waste represents an abundant biogenic calcium carbonate resource with potential for conversion into functional marine coatings. This study investigates a waterborne acrylic coating incorporating oyster-shell powder thermally treated at 600 °C and AZO@SiO2 core–shell nanoparticles. During a 12-month natural-seawater deployment at Anping Harbor, the three coated panels showed markedly lower visible macrofouling than the uncoated controls, although localized deposits were visible, whereas uncoated controls exhibited substantially greater visible surface coverage. Adjacent uncoated structures also showed spatial differences in fouling coverage, although these field observations do not establish the underlying biological mechanisms. Under laboratory illumination of 100 W m−2, the coated surface reached an apparent temperature of 42.1 ± 1.8 °C at 90 s, compared with 31.2 ± 0.8 °C for the uncoated control, corresponding to an apparent temperature difference of +10.9 °C. Preliminary gas chromatography–mass spectrometry (GC-MS) measurements revealed time-dependent and spatially varying analytical signals tentatively associated with low-molecular-weight amines. The proposed volatile-amine-mediated deterrence and photothermal antifouling mechanisms remain hypotheses requiring controlled biological validation. In preliminary 28-day leaching tests, Cu, Zn, Sn, and Fe concentrations were below the corresponding instrumental detection limits under the tested conditions. The estimated material and basic processing cost was approximately US$4.50 m−2, subject to further economic validation. These findings demonstrate the potential of oyster-shell waste valorization for developing functional marine coating systems while identifying the analytical, ecological, and durability investigations required before broader practical deployment.