Snail shell-derived hydroxyapatite on ant clay: a sustainable solution for phosphate sequestration and circular agricultural reuse.

In this study, a sustainable adsorbent comprising hydroxyapatite derived from snail shells and clay collected from ant nests was developed and used for the efficient removal of phosphate (PO4 3-) from aqueous solutions. The synthesized adsorbent was characterized using different characterization techniques like FTIR spectroscopy, XRD, SEM, TGA, PZC, and BET analysis to confirm its successful synthesis, morphology, thermal stability, surface charges, and surface area, respectively. The adsorbent was systematically investigated for phosphate adsorption by optimizing key experimental parameters, comprising contact time (25 min), adsorbent dosage (25 mg), initial phosphate concentration (5 mg L-1), solution pH (5), and temperature (40 °C). Furthermore, the adsorption kinetics followed the pseudo-second-order model (R 2 = 0.98), implying that the adsorption rate was controlled by the availability of active sites. The equilibrium data were best fitted to the Langmuir isotherm model (R 2 = 0.98), with the maximum adsorption capacity of 97.18 mg g-1. The thermodynamic analysis confirmed the endothermic (ΔH° > 0) and spontaneous (ΔG° < 0) nature of the adsorption process. Real sample analysis validated the practical application, with 84.1% removal of PO4 3-. Exploratory machine-learning modelling of 84 experimental observations showed that SVR-RBF most accurately predicted phosphate-removal efficiency (cross-validated R 2 = 0.789 ± 0.127), while Extra Trees regression provided the strongest prediction of adsorption capacity (cross-validated R 2 = 0.894 ± 0.210). The modelled response trends were consistent with the experimentally identified optimum conditions, supporting the use of data-driven methods for adsorption-process interpretation and future optimization. Additionally, the PO4 3--loaded adsorbent was evaluated as a fertilizer for wheat cultivation; it induced an enhanced growth performance compared to pristine ant clay, highlighting its potential for sustainable agricultural applications. Thus, this study established the adsorbent's efficacy in PO4 3- removal and its suitability for improving growth parameters, including germination rate, shoot/root development, and chlorophyll/protein content, resulting in improved phosphorus bioavailability.