Dual-purpose applications of magnetic phase-change microcapsules with crystalline-phase-tunable CaCO3 shell for waste heat recovery and heavy metal ion removal

Removing heavy metal ions and recovering waste heat from industrial wastewaters can provide a friendly environment for human being survival together with an encouraging opportunity for sustainable energy resource. To meet such a requirement of dual functions, three types of magnetic phase-change microcapsules were fabricated with n-docosane as a phase change material core and crystalline phase-tunable CaCO3 as a shell, in which Fe3O4 nanoparticles were embedded to endow the microcapsules with magnetic responsiveness. The surfactant plays a key role in the crystalline phase of their CaCO3 wall material, resulting in different morphologies for the magnetic microcapsules and generating a great influence on their thermal properties and adsorption performance. The magnetic microcapsules synthesized using different types of surfactants exhibit spherical, rhombohedral, and fusiform morphologies with phase-change enthalpies of 135.3, 87.1, and 48.7 J/g, respectively. Moreover, the magnetic microcapsules obtained maximum adsorption capacities of 703.1, 862.3, 924.0, and 514.7 mg/g for the removal of Cu2+, Cd2+, Cr3, and Fe3+, respectively. Such high adsorption capacities for removing heavy metal ions are superior to those of adsorbents reported in the literature. More importantly, the magnetic microcapsules exhibit outstanding phase-change reversibility and thermal cycling stability for waste heat recovery as well as good adsorption reusability and high efficiency for heavy metal ion removal. This study provides a promising strategy in the design and fabrication of bifunctional phase-change microcapsules for dual-purpose applications of heavy metal ion removal and waste heat recovery from various industrial wastewaters.