Iron-based composites for in-field phosphorus removal from agricultural drainage

Elevated phosphorus and nitrogen concentrations in agricultural drainage are responsible for widespread declines in water quality across freshwater and marine receiving environments worldwide. Removal of P at source is particularly important because its tight biogeochemical cycling with iron (Fe), leads to preferential P-retention (over N) across the aquatic continuum, particularly in lake basins. This paper introduces new Fe-C-based composites (Fe(III)oxide/akaganite) designed for use in edge-of-field mitigations such as denitrifying bioreactors, with the goal of intercepting and retaining P on-farm. Our Fe-functionalised media had a net positive charge in the favourable range for P adsorption under the typical pH conditions encountered in denitrifying bioreactors (pH 5.5 - 6). Although functionalized wood (Fe-WC) had significantly higher P adsorption capacity than functionalised biochar (Fe-PyOM), 65.8 mg g-1 and 31.4 mg g-1, respectively; this was of comparable magnitude to powdered akaganite (107 mg g-1), despite large differences in particle size. Batch and column adsorption experiments indicate a multi-layer adsorption mechanism for P removal on Fe-PyOM and Fe-WC, which was further investigated using XPS, confirming dominant electrostatic P binding by Fefunctionalised media below pH 6, but an increasing importance of ligand exchange (Fe-O-P) at pH 8, particularly for Fe-PyOM. Based on the prevailing geochemistry of influent solutions to DBRs (pe - -8, pH - 6), we suggest that Fe-PyOM may be able to continuously adsorb P over a long timespans without requiring replenishment, due to its higher surface area, stability, and multiple P binding mechanisms.