Phosphorus (P) advanced treatment by adsorption reduces the risk of eutrophication in natural waters and reservoirs. The impact of ubiquitous Ca 2+ on long-term P removal is critical in assessing the regeneration efficiency of one adsorbent, which is a vital indicator for cost-effectiveness. Given the critical role of lanthanum (La)-based composite materials in P removal, in this study, we unravel the long-term evolving role of Ca 2+ on phosphate removal by nanosized hydrated lanthanum oxides (HLO) confined in cross-linked polystyrene beads (HLO@201) over 20 adsorptionregeneration cycles and fixed-bed column runs, with a combination of macroscopic adsorption experiments, microscopic structural investigation, and theoretical calculations. The role of Ca 2+ gradually evolves from positive (5−70% higher than Ca 2+ -free group) to negative (18−41% lower than the Ca 2+ -free group) with ongoing cyclic runs of HLO@201, which is distinctive from the bulky HLO. The presence of Ca 2+ enhances P uptake by HLO@201 possibly through �La−P−Ca−P multiple complexation and Ca−P precipitation (i.e., hydroxyapatite, HAP) inside the polymeric host, which creates an antagonistic effect with HLO over time. The formed Ca−P precipitates may accumulate and encapsulate on the surface of HLO nanoparticles, which induce the formation of irreversible LaPO 4 •xH 2 O under nanoconfinement that deplete the active adsorptive sites. A two-step (acid wash + NaOH) regeneration method can partially recover the P removal performance of HLO@201. We envision that this study could be a cautionary tale for advanced treatment of P by adsorption, to inspire re-evaluation on the long-term performance of adsorption processes.

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