Calcium modification in food waste digestate derived granular biochar: unveiling synergistic mechanisms for phosphorus recovery

Abstract

Phosphorus (P) is a primary concern in wastewater. It is crucial to develop efficient adsorbents for long-term P removal from wastewater. In this study, we proposed a calcium carbonate (CaCO₃) enhanced granular biochar (BC) adsorbent by co-pyrolysis of food waste digestate, and systematically evaluated the adsorbent properties and adsorption mechanism. Material characterization has verified that in-situ Ca loading via 4–10 nm pore filling improves surface functional groups and enhances biochar yield. The adsorbent prepared with 10 % CaCO₃ achieved the best phosphorus removal capacity (18–20 mg-P/g-BC) versus unmodified counterparts (8–10 mg-P/g-BC). The adsorption followed Langmiur isotherm model and Pseudo-second-order kinetics. Calcium induced active sites enhanced the phosphate trapping with optimal phosphate removal occurring at pH 6–9. Modification with excessive CaCO₃ (20 %) induced structural fragility, and the CaP precipitate overformation hindered the further phosphate adsorption. Fixed-bed column adsorption revealed prolonged saturation thresholds (>125 h), and the adsorption fitted well with the Clark mass-transfer model (R² = 0.992). Notably, equilibrium between complexation and precipitation mechanisms was achieved at the 10 % CaCO₃ modification. This study establishes a strategy for optimizing granular P adsorbent systems through controlled mineral modification, thereby providing theoretical and practical foundations for full-scale phosphorus recovery deployment.