Construction of Di- and Tricarboxylic Acid Facilitated Lanthanum-Based Organic Frameworks on Agricultural Waste Biomass for Adsorption and Removal of Phosphorus from Aqueous Solution

Agricultural advancement boosts food production and economic growth but also causes solid waste and phosphorus pollution. Using agricultural waste to tackle pollution is a sustainable way to reduce environmental harm and promote resource recycling. In this study, lanthanum (La)-based metal-organic frameworks (MOFs) utilizing terephthalic acid (BDC) and 1,3,5-benzenetricarboxylic acid (BTC) as linkers─referred to as La-BDC MOF and La-BTC MOF, respectively─were synthesized on agricultural waste in the form of rapeseed pollen (PL) biomass, resulting in a refined support with enhanced functionality. A variety of advanced instrumental techniques were employed to investigate the formation of PL@La-BDC MOF and PL@La-BTC MOF. The maximum adsorption densities of PO₄^3- achieved under optimal conditions were recorded at 35.8 mg/g for PL@La-BDC MOF (pH 6.0, 25 °C) and 39.4 mg/g for PL@La-BTC MOF (pH 9.0, 35 °C). These values significantly surpass those reported for previously developed La-based adsorbents. The adsorption processes are best described by the pseudo-second-order model, indicating that chemisorption is the predominant mechanism. The presence of PO₄^3- on both the PL@La-BDC MOF and PL@La-BTC MOF following the adsorption process was verified through FT-IR and XPS analyses. In the case of PL@La-BDC MOF, the primary mechanism involves phosphorus anions in solution displacing hydroxyl groups that are coordinated to the central metal, La, through a ligand exchange process coupled with electrostatic attraction. For PL@La-BTC MOF, the main mechanisms involve electrostatic interactions and the chemisorption of carboxyl groups associated with the phosphorus. Moreover, both materials possess enhanced selective adsorption capabilities for phosphorus and exhibit robust anti-interference properties. This work presents an effective and eco-friendly biomass adsorbent for efficient phosphorus adsorption in solutions. It provides insights for utilizing agricultural waste in environmental remediation, enhancing the practical application of natural waste materials.