Intermittent aeration-driven iron cycling for the remediation of waste pit-sealing mud: metabolic mechanisms and model evaluation

The waste pit-sealing mud urgently needs to be remedied to ensure the sustainable development of sauce-flavored Baijiu. In this study, a pit-sealing mud quality evaluation model based on four physicochemical indicators was established and evaluated using 400 additional independent samples. Across eight mud states, the mean model scores showed close agreement with the corresponding sensory-reference scores (R² = 0.995). Subsequently, intermittent aeration (2 h at 3.6 L/min, once per week) was applied to induce iron cycle for the remediation of waste pit-sealing mud. The results showed that Fe(II) content decreased after intermittent aeration, and it increased again when aeration ceased and the system entered anoxic conditions. After 31 days, the removal efficiencies of organics and total nitrogen (TN) in the aerated reactors reached 53.0% and 51.6%, significantly higher than those in the control (43.1% and 15.3%, p < 0.05), respectively. Additionally, taxa previously associated with organic matter transformation and Fe(III) reduction showed higher relative abundances after intermittent aeration, while electron-donating and electron-accepting capacities increased by more than 30%. Metagenomic analysis indicated increased functional potential related to carbon metabolism, iron acquisition and transport, extracellular electron transfer, and energy metabolism. Furthermore, a coupled Fe-C-N metabolic pathway was proposed, in which organic matter degradation was linked to heterotrophic Fe(III) reduction, while a potential Feammox process may have contributed to nitrogen removal. According to the evaluation model, the score of remediated mud was 83.1, indicating effective remediation. This study provided a green strategy for the remediation of waste pit-sealing mud and offered a new idea for using limited iron to treat low C/N wastes.