Porous sulfur composites with built-in buffering unlock high-efficiency autotrophic denitrification across water alkalinity regimes

Sulfur-based autotrophic denitrification (SAD) offers an efficient, carbon-free route for nitrogen removal, yet suffers from the low bioavailability of elemental sulfur and poor adaptability under low-alkalinity conditions. This study presents a porous sulfur-based (PS) material synthesized via NaHCO₃-assisted melt-cooling granulation, integrating structural perforation and intrinsic buffering. NaHCO₃ served both as a perforation agent during fabrication and a sustained alkalinity source during use. Structural characterization revealed primary perforation formed interconnected macropores for microbial colonization, while secondary perforation, driven by gradual NaHCO₃/Na₂CO₃ dissolution, expanded porosity during operation. Performance testing showed PS materials achieved nitrate removal rates 1.3–3.8 times those of raw sulfur across both high- and low-alkalinity conditions. The volume of macropores greater than 500 nm exhibited a stronger correlation with the denitrification rate than the specific surface area (R² = 0.85 vs. 0.65). PS materials also enriched Thiobacillus and sustained alkalinity release over their lifespan. A sulfur-alkalinity balance analysis provided a framework for material selection under different influent conditions. From an engineering perspective, PS materials could reduce biofilter volume by up to 74 %, lowering capital cost and land use despite a modest rise in operational cost. These findings advance the structure-function understanding of functional media and support scenario-specific material strategies for scalable SAD application.