Facet-Specific Dissolution–Precipitation at Struvite–Water Interfaces

One beneficial approach to phosphorus recovery from wastewater is through struvite (MgNH₄PO₄·6H₂O) crystallization, which could potentially be used as a slow-release fertilizer. However, it is often ignored that the reactivity and fate can be effectively influenced by naturally abundant metal ions, such as Ca²⁺ in soil solutions, which results in the formation of sparingly soluble calcium phosphate precipitates on dissolved struvite crystal surfaces. Here, we use in situ atomic force microscopy coupled with a fluid reaction cell to observe interfacial dissolution–reprecipitation reactions of Ca²⁺-bearing solutions with distinct struvite surfaces. Our results show the formation of acidic amorphous calcium phosphate and its subsequent transformation to monetite (CaHPO₄) crystals on the (011) face of struvite at a wider pH range; by contrast, the occurrence of basic amorphous calcium phosphate and its subsequent transformation to whitlockite (Ca₂₉Mg­(HPO₄)₃(PO₄)₁₈) and β-TCP (β-Ca₃(PO₄)₂) is observed on the (001) face of struvite under acidic and alkaline conditions, respectively. Owing to Mg²⁺ cations possessing a single oxygen deficit relative to the saturation coordination on the (001) surface, the {011} faceted surfaces most likely control an invariant Ca/P atomic ratio (at 1.0) in amorphous precursor phases through the formation of phosphate-bridged ternary complexes (Mg–P–Ca), which produce CaHPO₄⁰ precipitates rather than whitlockite and β-TCP. Surface-specific dissolution of struvite is thus linked to the simultaneous growth of different calcium phosphate phases through the formation of precursors with distinct Ca/P atomic ratios, which are likely to be key factors in controlling the interfacial reactivity and fate of struvite under varied environmental solution conditions.