Ferric iron incorporation promotes brushite hydrolysis and enhances cadmium immobilization

Dissolution-precipitation processes on the surface of brushite (dicalcium phosphate dihydrate, DCPD) control the migration and transformation of potentially harmful elements (PHEs). The incorporation of impurities could affect the properties of DCPD and its interactions with PHEs. In this study, we synthesized Fe³⁺-bearing DCPD via coprecipitation and investigated the influence of Fe³⁺ incorporation on the crystal structure, hydrolysis process, and Cd removal performance. Fe-bearing DCPD had lattice expansion due to the coupled substitution of Fe³⁺ and NH₄⁺ for Ca²⁺. Therefore, the Cd removal performance of Fe-DCPD was enhanced, with a maximum Cd uptake capacity of 431.6 mg/g, which is 1.77 times that of Fe-free DCPD (244.4 mg/g). Furthermore, Fe-DCPD also exhibited a faster hydrolysis rate, which was up to 2.67 times that of Fe-free DCPD and accelerated Cd's transfer to the stable host mineral, hydroxylapatite. Cd was first caught by the DCPD surface in a weakly crystalline form and then incorporated into the hydroxylapatite structure during crystallization. Based on the X-ray photoelectron spectroscopy and thermogravimetric analysis results, we concluded that the decrease in interstitial water due to Fe incorporation was responsible for accelerating hydrolysis and enhancing Cd immobilization. In all, the incorporation of Fe³⁺ into DCPD could promote its transformation and improve its Cd uptake capacity. Our results suggest that Fe-DCPD could be a promising candidate for environmental remediation.