Unveiling the protective role of low-intensity ultrasound pre-acclimation enhances partial nitrification against rare earth element La (III) shock: EPS remodeling and antioxidant defense amplification

Partial nitrification (PN) is a promising energy-efficient technology for treating rare earth tailings wastewater; however, its application is limited by the inhibitory effects of residual rare earth ions. This study investigated whether low-intensity ultrasound pre-acclimation could enhance PN sludge tolerance to acute La(III) shock and elucidated the underlying protective mechanisms. Ultrasound pre-acclimation substantially increased the La(III) tolerance threshold, with the half-inhibitory concentration (IC 50 ) increasing from 71.48 to 170.10 mg/g VSS. This enhanced tolerance was achieved through two coordinated protective mechanisms. Extracellularly, ultrasound pre-acclimation remodeled the EPS composition, promoting polysaccharide secretion under low-to-moderate La(III) shock and preserving protein content and ordered secondary structures under high La(III) shock, which collectively restricted intracellular La(III) accumulation. Intracellularly, ultrasound pre-acclimation enhanced antioxidant defense responses: superoxide dismutase and catalase activities, as well as glutathione content, increased approximately four-fold compared with the control at 300 mg/L La(III), thereby attenuating the shock-induced increase in reactive oxygen species. FTIR analysis of whole sludge further confirmed that ultrasound-acclimated sludge maintained functional group integrity under La(III) shock. Both protective mechanisms operated over a similar concentration range, roughly around 300 mg/L. At 500 mg/L La(III), the protective effects were weakened across all evaluated indicators. These findings demonstrate that low-intensity ultrasound pre-acclimation is an effective strategy for enhancing PN sludge resistance to La(III) shock and provide insights into its potential application in biological nitrogen removal from rare earth tailings wastewater.