A novel synthetic route to hydroxyapatite-zeolite composite material from steel slag: investigation of synthesis mechanism and evaluation of physicochemical properties

Steel slag is a commercial waste material mainly consisting of SiO2, Al2O3 and CaO, the former two chemicals being major components of zeolites and the latter a major component of hydroxyapatite (HAP). A hydroxyapatite-zeolite composite material (HAP-ZE) was successfully synthesized from steel slag by adding appropriate chemical reagents, H3PO4 and NaOH, via aging at 363 K for approximately 2 days. The synthesis mechanism and structural properties were clarified by detailed analysis using XRD, FT-IR, SEM, EDX, elemental mapping, and N-2 adsorption-desorption measurements. The Ca and Mg components were chemically reacted with phosphate in the early stages of aging, being precipitated as Mg-substituted HAP with (Ca + Mg)/P = 1.67. After 2 days of aging, well-crystallized HAP and faujasite-type zeolite (Na type X-zeolite with SiO2/Al2O3 = 2.4) were separately formed via a non-simultaneous crystallization process. The over-run in aging time led to phase transformation from FAU-zeolite to Pl-zeolite. The minor components in steel slag such as Fe and Mn had little effect on the synthesis of HAP-ZE; however, the inherent SiO2/Al2O3 ratio in steel slag led to a lower yield from the zeolite phase. Furthermore, from the adsorption assessment of volatile organic compounds (VOCs), fatty acid and protein, the HAP-ZE synthesized under optimum conditions was found to have adsorption properties comparable to those of pure zeolite and HAP.