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

Abstract

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.