Lanthanum Metal-Organic Frameworks as Binding Agent in Diffusive Gradients in Thin Films (DGT) for Phosphate Monitoring: Influence of pH and Competing Anions

Phosphate mobility and bioavailability play critical roles in aquatic ecosystems, yet conventional monitoring methods are limited by sample disturbance and poor representation of in-situ conditions. The Diffusive Gradients in Thin Films (DGT) technique overcomes these limitations by enabling passive, in-situ measurement that accurately reflects natural solute diffusion and sediment resupply dynamics. This study introduces lanthanum metal-organic frameworks (La-MOF) as a novel selective binding agent in a DGT for in situ phosphate monitoring. La-MOF was synthesized via solvothermal using DMF (La-MOF 1) and mixed DMF/water (La-MOF 2), then characterized by XRD, FTIR, SEM-EDX, and BET analysis. Structural characterization confirmed successful framework formation for both materials. La-MOF 2 exhibited improved crystallinity, more homogeneous morphology, higher specific surface area (84.96 m2 g-1) and larger pore volume (0.090 cc g-1) compared to La-MOF 1. The selectivity performance of La-MOF DGT was evaluated across pH 2-11 and in the presence of competing anions (NO3-, SO42-, and CO32-). Optimal phosphate uptake occurred at pH 4-7, corresponding to the predominance of H2PO4- species. The inhibitory effect on phosphate uptake followed the order: NO3⁻ > CO3²⁻ > SO4²⁻. These findings demonstrate the potential applicability of La-MOF as a selective binding agent for DGT-based phosphate monitoring in aquatic environments.