Post-regeneration neutralization enables stable phosphate adsorption-desorption in Ca- and NOM-rich waters

Residual alkalinity left after alkaline regeneration is a critical yet often overlooked factor controlling the long-term performance of iron-oxide adsorbents for phosphorus (P) removal and recovery. In porous adsorbents, incomplete neutralization can maintain a local alkaline environment, promoting calcium (Ca)-based precipitation and performance deterioration. This study evaluated post-regeneration neutralization of the iron oxide adsorbent GEH in Ca- and natural organic matter (NOM)-containing water matrices. Three neutralization conditions were compared: rapid rinsing, thorough washing, and acid-assisted neutralization. Rapid rinsing and thorough washing left residual alkalinity within GEH, driving Ca-based precipitation and progressive loss of adsorption capacity and regeneration efficiency. In contrast, acid-assisted neutralization largely reduced residual alkalinity, suppressed precipitation, and maintained adsorption-dominated P removal. Under acid-assisted neutralization, GEH sustained stable P removal of 4.51-5.83 mg/g with 88%-96% regeneration efficiency over 12 cycles in Ca- and NOM-containing systems. These findings identify post-regeneration neutralization as a preventive control step for residual alkalinity, rather than a delayed corrective treatment for accumulated precipitates, providing a practical strategy to improve the stability of adsorption-based P removal and recovery.