Bio-Inspired Crystallinity Regulation of Calcium Phosphate: Structure-Dependent Adsorption Mechanisms and Fluorescence Detection for Rare Earth Recovery

The adsorption performance and mechanisms of calcium phosphate are highly dependent on its crystallinity, yet precise control over crystallinity remains challenging due to the instability of low-crystalline phases. Inspired by the Mg2+-aspartic acid synergy in crustacean molting, we developed a bio-inspired strategy using polyacrylic acid and Mg2+ to finely regulate calcium phosphate crystallinity. Based on this strategy, three sodium alginate (SA) composite hydrogels were fabricated for Eu3+ and Tb3+ recovery, including SA/HAp (crystalline hydroxyapatite), SA/Half (semi-crystalline), and SA/ACP (amorphous). The crystallinity of calcium phosphate strongly influenced adsorption performance, with SA/ACP showing the highest capacities: 401.61 mg/g for Eu3+ and 471.68 mg/g for Tb3+, 79 and 57% higher than those of SA/HAp, respectively. All composite hydrogels followed pseudo‑second‑order kinetics and the Langmuir model, while adsorption affinity and intraparticle diffusion rates increased as crystallinity decreased. Lower crystallinity also increased the specific surface area, mesopore volume, and solubility of calcium phosphate, thereby facilitating ion transport and active-site accessibility. Thermodynamic analysis indicated spontaneous, endothermic, and entropy-increasing processes, with SA/ACP showing the most significant temperature response. Mechanism studies demonstrated that adsorption involves surface complexation, dissolution-precipitation, and ion exchange, with the contribution of each pathway varying with crystallinity. ACP underwent nearly complete transformation to phosphates of REEs through dissolution-precipitation and ion exchange, while crystalline hydroxyapatite showed a limited surface reaction dominated by surface complexation. SA/ACP also exhibited excellent fluorescence detection for Eu3+ and Tb3+ in acidic phosphor leachates. Fixed‑bed column tests demonstrated SA/ACP's practical potential, with removal efficiencies exceeding 90% throughout the treatment of approximately 2500 (Eu3+) and 2850 mL (Tb3+) of simulated wastewater. This bio‑inspired approach provides fundamental insights into structure-property relationships in calcium phosphate‑based adsorbents.