Synergistic Runoff Particulate‐Bound Phosphorus Extraction by Oxalate and Citrate Ligands in a Mixed Aqueous Extractant System

Recovering nonpoint phosphorus (P) losses in particulate runoff is considered a significant step toward closing the P loop for nutrient and water sustainability. The objectives of this study are to examine how process conditions affect P extraction efficiency using an abiotic 2:1 mM oxalate/citrate extractant mixture and investigate the sediment P extraction kinetics in this system in comparison to a pure citrate extraction process. High P removal and solubilization are favored at low sediment loading (5 g/100 mL), low initial pH (3), and high initial organic acid loading (75 mM), with P recovery efficiencies up to 99.6% achieved under these conditions, similar to a pure citrate system. Initial and equilibrium extractant pH levels near oxalate and citrate pKa 2 s (4.19 and 4.74, respectively) appear conducive for optimal and/or synergistic oxalate ligand exchange and citrate ligand‐induced particulate P release. P solubilization kinetics and P speciation dynamics analysis indicate better performance by the oxalate/citrate mixture through a higher initial P solubilization rate by rapidly extracting Fe‐P from the sediments within the first 30 min of extraction, indicating potential synergism between the oxalate and citrate ligands. From a process development standpoint, the results suggest that operating the extraction under fed‐batch or semi‐continuous modes with pH control may maximize runoff particulate P recovery efficiency using a mixed low molecular weight organic acid (LMWOA) extractant.