Waste Shell Valorization for Phosphorus Recovery: Comparative Assessment of Cockle Shell- and Eggshell-Derived...

Phosphorus pollution from municipal and agricultural effluents remains a primary driver of freshwater eutrophication globally. Yet, conventional removal technologies permanently immobilize phosphorus in metal-laden sludge, precluding meaningful nutrient recovery. This study valorizes biogenic calcium carbonate wastescockle shell (Anadara granosa, aragonite-based) and chicken eggshell (Gallus gallus domesticus, calcite-based)as precursors for calcium oxide adsorbents in powder and granular configurations. Six adsorbent variants were synthesized and characterized using BET, SEM-EDS, XRF, ICP-OES, and ATR-FTIR to elucidate structure-composition-performance relationships. Batch adsorption experiments conducted across pH 3-11, concentrations of 0.2-500 mg P/L, and contact times of 0-72 h revealed chemisorption consistent with hydroxyapatite (HAP) precipitation, supported by convergent spectroscopic and elemental evidence, following pseudo-second-order kinetics and Langmuir isotherms. CS-900 achieved a maximum Langmuir capacity of 15.15 mg/g, exceeding ES-900 (10.65 mg/g) by 42.3% (p = 0.008), a difference consistent with synergistic physicochemical factors, including higher strontium content, crystallographic differences between aragonite and calcite precursors, and elevated surface area in aragonite-derived CaO. Cement-bound granules (10 mm diameter; 28.1-33.8 kgf crushing strength) retained 48-69% of powder capacity while enabling mechanically stable continuous-flow operation. Fixed-bed column experiments yielded a CS-900-G dynamic capacity of 10.43 ± 0.8, which exceeded batch predictions, a finding consistent with enhanced calcium dissolution under sustained concentration gradients; Thomas model analysis provided good empirical agreement with diffusion-limited kinetics (R ² > 0.96). Although alkaline regeneration was ineffective due to the HAP thermodynamic stability, spent adsorbents accumulated phosphorus equivalent to 21.5 wt % P₂O₅, comparable to commercial bone meal fertilizers, supporting agricultural reuse as a quantifiable circular phosphorus recovery pathway. This system demonstrates proof-of-concept potential as a chemical-free polishing stage for decentralized applications, pending validation under real wastewater conditions. To our knowledge, this is the first systematic comparison of aragonite-versus calcite-derived CaO adsorbents across powder and granular configurations under continuous flow conditions.