Molecular insights and thermodynamic feasibility of phosphate adsorption on Ca-biocomposites using a simplified carbon structure

The removal of phosphorous from water sources is a critical challenge in mitigating water eutrophication. Adsorption using Ca-biocomposite-derived materials has proven to be highly effective for phosphorus removal. These biocomposites contain Ca, CaO, and Ca-(hydr)oxide species, which form Ca-P apatite phases, a potential fertilizer, holding promise for phosphorus recycling and promoting a circular economy. Density Functional Theory calculations were conducted to gain molecular insights into the thermodynamic feasibility of calcium-based adsorbents. Four models were used, viz. Ca²⁺, CaO monomer and dimmer, and Ca-(hydr)oxide, all embedded in a carbon matrix. Several binding modes were evaluated for [H₂PO₄]^- and [H₂PO₄.6H₂O]^-, including monodentate mononuclear, bidentate mononuclear, and bidentate binuclear denticities. Adsorption, enthalpy, and Gibbs free energy changes were used as descriptors, together with Natural Bond Orbitals analysis. The findings indicate that [H₂PO₄.6H₂O]^- adsorption is thermodynamically favored primarily on the CaO dimmer, followed by the CaO monomer, and finally on Ca²⁺, suggesting a preference for binding phosphate through a monodentate mononuclear mode. The [H₂PO₄]^- adsorption on the Ca-(hydr)oxide model was found to resemble the system's pH considering the H₂O/OH ratio, an approximation of acid, intermediate, and basic pH conditions. Our research demonstrated the phosphate adsorption feasibility across a range of pH conditions, providing a solid foundation for further analysis such as infrared and X-ray photoelectron spectroscopy. The Ca-(hydr)oxide model effectively simulates interactions between phosphate species and calcium-based adsorbents, approximating real environments. It enhances understanding of adsorption across various chemical environments, including the pH effect, and aligns with observed structural changes during phosphate adsorption. By combining theory with practical applications, this model aids in comprehending phosphate removal processes, guiding adsorbent optimization, and environmental strategies like eutrophication mitigation.