Impact of Sponge Iron Proportion in Bioretention Media on Decontamination Performance and Biological Characteristics

Sponge iron, an emerging bioretention medium modifier, and the appropriate proportions play a crucial role in runoff purification and biological growth. However, the mechanism by which sponge iron proportions affect bioretention systems remains unclear, thereby hindering its effective application. In this study, five bioretention systems were constructed to systematically investigate the impact of sponge iron on decontamination performance, plant physiological responses, and microbial community characteristics. As the dosage of sponge iron increased, the average removal rates of ammonia nitrogen (NH4+-N), nitrate nitrogen(NO3−-N), total nitrogen (TN), total phosphorus (TP), and chemical oxygen demand (COD) initially increased and then stabilized or slightly declined. Removal rates reached 94.34%–98.62% (1:8), 91.05%–95.76% (1:2), 87.90%–95.70% (1:6), 96.00%–96.60% (1:8), and 95.42%–98.58% (1:2), respectively, under various influent conditions. Although sponge iron induced oxidative damage to plants, moderate doses stimulated the activity of antioxidant enzymes, thereby enhancing plant self-protection mechanisms. The proper addition of sponge iron helped maintain optimal microbial abundance and diversity despite its potential adverse effects on certain functional bacteria. Overall, the recommended sponge iron addition ratio of 1∶6 achieved relatively excellent purification effects and biological activity.