Enhanced Stormwater Treatment via Thermally Modified Steel Slag-Based Bioretention System: Performance Evaluation and Mechanistic Insights

Conventional bioretention systems face challenges in effectively removing dissolved nutrients, heavy metals, and emerging contaminants from stormwater runoff. This study investigates the application of thermally modified steel slag (700 °C) as a functional bioretention matrix for comprehensive stormwater purification. Three pilot-scale systems were evaluated over 120 days: Control (biochar-zeolite), Unmodified (raw steel slag-biochar-zeolite), and Modified (thermally modified steel slag-biochar-zeolite). The modified system demonstrated superior and stable removal efficiencies for NH4+-N (95.3 ± 1.3%), TN (85.7 ± 1.8%), TP (90.5 ± 1.5%), Cu2+ (96.1 ± 0.7%), Cr6+ (90.5 ± 1.2%), Pb2+ (92.2 ± 1.1%), enrofloxacin (65.6 ± 2.1%), and norfloxacin (62.6 ± 2.4%). Performance remained robust under varying hydraulic conditions, with high removal maintained across rainfall return periods (0.5–2 years) and antecedent dry periods (2–8 days). Mechanistic investigations revealed synergistic effects: (1) Enhanced physical adsorption through increased surface area (2.338 m2/g) and pore volume (0.109 cm3/g); (2) Chemical precipitation via Ca2+/Fe3+ release at alkaline pH (8.2–8.5); (3) Enriched microbial communities with 35% higher Shannon diversity, particularly Hydrogenophaga (12.3%) for autotrophic denitrification using Fe2+ as electron donor. The modified slag matrix creates a “triple-barrier” removal mechanism combining physical, chemical, and biological processes, offering an efficient solution for multi-pollutant stormwater treatment. This study demonstrates that thermally modified steel slag represents a high-performance, cost-effective bioretention matrix for comprehensive stormwater treatment while promoting industrial byproduct utilization and aligning with circular economy principles.