Nanoconfinement Decouples Calcium-Enhanced Phosphate Capture from Calcium Phosphate Crystal Maturation

Calcium phosphate (Ca-P) scaling, which is ubiquitous in nature and in water systems, has been shown to interfere with the capture of phosphate by ferric oxide-based sorbents, thereby deteriorating their decontamination performance in repeated cycles. Here, we show that a specifically designed gel-type nanocomposite, namely HFO@N201, enables Ca-enhanced phosphate sequestration while restricting Ca-P scaling via nanoconfinement. Increasing the initial Ca2+ concentration from 0 to 160 mg/L improved phosphate uptake from ∼4.5 to ∼6.2 mg/g in the first cycle. After 20 adsorption-regeneration cycles, phosphate uptake remained at ∼6.4 mg/g in the Ca-containing system, twice that of ∼3.2 mg/g in the Ca-free system. In fixed-bed tests, the presence of Ca2+ enhanced the working capacity from ∼1350 to ∼1800 bed volumes, which could be observed again in the subsequent cycle. Asynchronous release of Fe, P, and Ca during acidic elution of P-loaded HFO@N201 indicates heterogeneous and nonstoichiometric Ca-associated phosphate retention rather than formation of a mature bulk Ca-P phase. XRD, TEM, EXAFS, and Mössbauer spectroscopy further show that the retained Ca-P species remained weakly ordered and that the nanosized ferrihydrite-like Fe phase persisted during cyclic operation. Thermodynamic analysis, classical nucleation theory, and molecular dynamics simulations further support that the nanoconfinement structure of HFO@N201 (∼2 nm) creates a low-mobility hydration environment that restricts solvent exchange, ion rearrangement, and long-range ordering, thereby stabilizing Ca-P species in a weakly ordered interfacial state. Our work demonstrates nanoconfinement as a strategy to exploit Ca-enhanced phosphate capture while reducing Ca-P scaling in regenerable sorbents.