Temporal sulfur redox reprogramming enables control-oriented intensification of sulfur-based autotrophic denitrification

Sulfur-based autotrophic denitrification (SAD) is widely used for nitrate removal from low-carbon wastewaters, yet its capacity is difficult to tune because the electron donor is a solid phase with slow interfacial supply. Sulfur disproportionation (SDP) can emerge under nitrate-depleted conditions and is usually treated as a failure mode due to sulfide formation. Here, we show that SDP can be converted into a controllable “charging” step and coupled with SAD in time to create an endogenous electron-donor buffer that intensifies denitrification without reactor reconfiguration. In laboratory packed beds (70 d), three SDP inductions (12–42 h) generated an immediate post-switch increase in denitrification rate from a baseline of 0.49 ± 0.07 to 0.71–1.28 kg-N/m³/d (1.45–2.61-fold). Enhancement persistence expanded with SDP intensity, lasting 1, 5, and 12 d as maximum sulfide increased from 10.5 to 33.9 mg/L. Sulfate-based stoichiometry captured phase-dependent sulfur conversion, with an excess sulfate fraction of 24.8–68.6% during SDP. Interfacial evidence supported reversible reduction during SDP, including a 0.99 eV decrease in Fe 2p_3/2 binding energy and an increase of Mössbauer-resolved Fe(II) from 73% to 81%, accompanied by protein-normalized accumulation and depletion of reduced sulfur and iron-associated species. Microbial communities restructured directionally under switching without pronounced enrichment of obligate disproportionators. Pilot-scale validation (40 m³) reproduced the intensification trend: temporal operation reached 1.87 kg-N/m³/d (3.9-fold of baseline 0.48 ± 0.04 kg-N/m³/d) and delivered 2.3-fold higher rates at 1.1 kg-N/m³/d loading while effluent sulfide declined to below detection as nitrate loading increased. This work demonstrates that temporal coupling of sulfur disproportionation and sulfur-based autotrophic denitrification can create an internal sulfur buffer, which may offer potential for enhancing denitrification resilience under fluctuating nitrate conditions.