The role of Ca2+ in the improvement of phosphate adsorption in natural waters: Establishing an environmentally friendly La/Ca bimetallic organic framework

Modified metal-organic framework (MOF) materials are promising adsorbents for phosphate removal in aquatic environment. Herein, a high-efficiency and eco-friendly La/Ca composite (La/Ca-BTC) was designed by calcining La/Ca MOFs for phosphate adsorption. Batch adsorption experiments showed that La/Ca-BTC-3/1 (La: Ca molar ratio of 3: 1) had an excellent phosphate sorption capacity of 101.01 mg P/g, and could also maintain relatively high adsorption in the range of pH 4-8. Anion coexistence experiments showed that, except for carbonate ions, common anions have little effect on adsorption. X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) analysis indicated that oxygen vacancies formed in the La/Ca-BTC, probably by metal doping. The density functional theory (DFT) calculation showed that oxygen vacancies could affect the orbital hybridization energy during phosphate adsorption by changing the state density, reducing the bond energy barrier for phosphate adsorption, thereby enhancing the adsorption effect of La/Ca-BTC. Phosphate adsorbents generally incur severe environmental risk by their gradual release of metal ions due to changes in water quality, especially where there is high natural organic matter (NOM). The DFT calculation further demonstrated that Ca2+ in the La/Ca-BTC was more inclined to combine with humic acid (HA) than La3+. Therefore, due to the introduction of Ca2+, La/Ca-BTC exhibited lower La-release in the presence of HA than La-BTC, which could be reduced by about 52.04%. Furthermore, La/Ca-BTC had the potential to simultaneously remove NOM which has important implication for aquatic remediation. These results are of great significance for the development of environmentally friendly phosphate adsorbents.