Optimization and characterization of thermally modified oyster shells for phosphate removal from wastewater: Insights supported by response surface methodology

Thermally modified oyster shells, which comprise reactive calcium oxide, show great promise as bio-based adsorbents for phosphate removal from wastewater. However, consensus is lacking on the effects of thermal conversion methods (pyrolysis and calcination) and treatment temperature on phosphate removal performance. Accordingly, thermal conversion methods and temperatures were first screened to identify the optimal material, followed by response surface methodology (RSM) optimization of removal conditions. Screening results showed that pyrolysis and calcination had comparable effects, while only oyster shell treated at 900 °C sustained removal efficiencies above 90% at an L/S of 10000, highlighting temperature as the dominant factor. RSM optimization further showed that the calcined oyster shell at 900 °C achieved an adsorption amount of 304 mg P/g with 95% removal efficiency at an L/S of 16000. Thus, only 63 g of calcined oyster shell is required to treat 1 m3 wastewater containing 20 mg P/L within one day. XRD, FTIR, and thermodynamic analyses indicated that phosphate removal was primarily through precipitation as calcium hydroxyapatite. Compared to other shell-based adsorbents, calcined oyster shell at 900 °C in this study exhibits superior phosphate removal performance, providing theoretical support and practical insights for wastewater treatment.