Simultaneous Removal of Phosphate and Nitrate from Stormwater via Biofilters Co-amended with Biochar and Iron-Rich Mining Residuals

Removal of nitrate and phosphate from runoff in a single treatment system is challenging because the two nutrients have different removal mechanisms. Here, we performed column tests under intermittent flow conditions to simulate simultaneous nitrate and phosphate removal in passive stormwater filtration systems using mixtures of sand, wood-based biochar (red pine woodchips pyrolyzed at 550 °C), and two natural iron-bearing materials generated as residuals from taconite mining (rich in iron oxide (IO) or siderite (SR)). Experiments were performed in a greenhouse in three phases (i.e., two dosing periods separated by an extended dry period) to evaluate how performance responds to antecedent dry periods under representative seasonal temperature variations. Mixtures with iron-bearing materials demonstrated effective phosphate retention, in contrast to biochar/sand mixtures, which poorly retained phosphate. Nitrate was retained to some degree in all biochar-amended columns during the first dosing period but was largely released during the second dosing period. Interestingly, iron/biochar columns were more resistant to nitrate release, particularly the SR/Biochar columns. 16S rRNA sequencing revealed that the used SR/Biochar media possessed a distinctive copresence of abundant Nitrosomonadaceae and Vicinamibacteraceae bacteria that have been linked with ammonia oxidation and iron metabolism. Accelerated leaching of Fe2+ from the SR material, which has been reported to influence shifts in dominant nitrate reduction pathways (i.e., denitrification vs dissimilatory reduction of nitrate to ammonia), in co-occurrence with biochar-facilitated electron shuttling processes, provides a potential rationale for the establishment of this distinctive microbial consortia. These results suggest that mixtures of wood-based biochar and iron-rich minerals may synergistically support microbial communities that enhance beneficial nitrogen- and iron-cycling processes while simultaneously adsorbing phosphate.