The potential of anthraquinone-2-sulfonate (AQS) for "shell breaking" of iron-coated denitrification granular sludge

This study investigated a cyclic on/off strategy using anthraquinone-2-sulfonic acid (AQS) to overcome sludge surface passivation caused by long-term operation of zero valent iron-based denitrification process. Results demonstrated that the nitrate removal efficiency (NRE) of reactor reached 87.94 ± 0.71 % with introduction of 100 mg L^-1 AQS for 7 days in the first cycle, which was 1.30 times higher than that of no aid of AQS phase (P0). Once AQS was absence in cycle, the NRE was 73.52 ± 2.66 %, which was also 1.09 times higher than that of P0. During cycle two, as reintroduction of AQS, NRE enhanced to 84.86 ± 5.63 %, which was 1.27 times higher than that of P0. Meanwhile, with absence of AQS, NRE maintained 71.49 ± 0.02 %, which was 1.06 times higher than that of P0. Mechanistic analysis revealed that AQS released the embedded iron ion and then which fed back to denitrification process, migrated to extracellular polymeric substances (EPS) and stimulated its additional secretion, increased the relative abundance of Thauera, Subgroup_7, A4b and Hydrogenedensaceae, and promoted the re-enrichment of Pannonibacter and Lentimicrobium after shell breaking. These effects collectively restored the mass transfer rate of denitrifying bacteria, and confirmed that AQS disintegrated the iron shell, released bioavailable iron ion to support iron-based autotrophic denitrification, and reactivated near-inactive sludge. This study revealed new insight for mitigating sludge surface passivation in iron-mediated denitrification systems treating low carbon-to-nitrogen ratio wastewater.