Iron- and Calcium-Based Mineral Media for Phosphorus Removal from Synthetic Phosphate Solutions: Implications for Agricultural Runoff Interception

Agricultural phosphorus losses create a need for simple and sustainable reactive media suitable for decentralized interception. This study compared unmodified pyrite, iron filings, and limestone for phosphorus removal from low-concentration synthetic phosphate solutions using dosage, apparent thermodynamic, nonlinear isotherm, kinetic, pH, SEM, and binary-mixture analyses. Based on the tested dosage range, 60 g/L was selected for pyrite and 40 g/L for iron filings and limestone. Pyrite showed an exothermic apparent response (ΔH° = −8.12 kJ/mol), whereas limestone and iron filings were endothermic (ΔH° = 11.75 and 19.99 kJ/mol). Nonlinear Freundlich fitting provided a slightly better description than Langmuir fitting, with R2 values of 0.9867, 0.9821, and 0.9648 for pyrite, limestone, and iron filings, respectively. Linearized pseudo-second-order fitting characterized the short-term phosphorus-removal kinetics of pyrite and limestone, while iron filings exhibited non-monotonic behavior. The 1:1 iron filings–limestone mixture maintained phosphorus removal above 99% across 10–100 g/L, with the highest removal at 40 g/L. The pH and SEM observations supported material-specific reaction environments and surface transformations. These batch-scale results identify unmodified Fe- and Ca-based media as candidates for further evaluation in sustainable agricultural phosphorus management under real runoff and continuous-flow conditions.