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

Abstract

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.

Keywords

• DGT
• lanthanum
• MOF
• phosphate