Fabrication and mechanism of La/Al bimetallic organic frameworks for phosphate removal

Lanthanum (La) is constantly used in phosphorus removal due to its high affinity with phosphate. Herein, a novel La/Al bimetallic organic framework was developed to improve the usage efficiency of La species. The effects of metal ratio, pH value and competitive anions were investigated, through which the adsorption mechanism was further explained. Results showed that when the doped La:Al ratio was 1:3, the absorbent (La1Al3-BTC) achieved the most excellent adsorption capacity of 210.3 mg P·g−1 and usage efficiency (P/metal ratio = 1.09). La1Al3-BTC can adapt to the wide pH range and multitudinous coexisting ions in real water, and its adsorption process was in accordance with Langmuir and pseudo-second-order models. Surface morphology of materials transformed from rod-like crystal to amorphous structure as the increase of Al doping. La replaced some Al atoms and embedded in Al-BTC skeleton. The specific surface area raised from 9.27 m2·g−1 of La-BTC to 257.01 m2·g−1 of La1Al3-BTC. The main mechanism for the adsorption of phosphate was the inner-sphere complexation occurred in the La active sites. Electron migration between different metals enhanced the adsorption process. Density-function theory (DFT) calculation well matched the experimental phenomenon. Overall, the development of La1Al3-BTC presented new potential applications for enhanced phosphorus removal and provided new insights for the design of La-based adsorbents.