Phosphorus (P) adsorption from aqueous solutions is usually evaluated by monitoring the P concentration and employed kinetic models. In this work, three adsorbents obtained from eggshell (ES) and eggshell mixed with palm mesocarp fiber (ESF-1:1 and ESF-1:10) at different Ca(OH)2/CaCO3 compositions were evaluated, and the Ca–P species formed monitored as a function of time deconvoluting Fourier Transform Infrared (FTIR) spectra. At 0.25 h the ESF-1:10 (Ca(OH)2: 26.2 wt%) exhibited better adsorption performance of 35 mgg−1 while ESF-1:1 and ES (Ca(OH)2: 2.8 and 3.0 wt%) showed 26 and 4 mgg−1, respectively. Characteristic PO43− bands in apatite were corroborated by XRD and FTIR. It was found that the role of Ca(OH)2 in the adsorption ends before 0.25 h, and thereafter CaCO3 becomes the phase responsible for the removal of orthophosphate H2PO4−/HPO42−/PO43− ions. The results indicate a direct ligand exchange of CO32− for PO43− that takes place while increasing the apatite crystallinity. On the other hand, the P adsorption process is also dependent on P concentration. At low P concentrations, characteristic bands of PO43− in apatite were observed in FTIR, while at high concentrations, characteristic bands for adsorbed HPO42− were obtained. The obtained results give a relevant role to CaCO3 in P adsorption. Kinetic analysis for Ca-based biocomposites showed that the Avrami order kinetic model fits better for the adsorbents. For P adsorption isotherm process the Langmuir's isotherms showed a good fit, with a maximum adsorption capacity of 90.8, 134.0, and 67.9 mgg−1 for ES, ESF-1:1, and ESF-1:10, respectively. © 2021