Synthesis of a Se0/calcite composite using hydrothermal carbonation of Ca(OH)2 coupled with a complex selenocystine fragmentation.

In this study, the hydrothermal carbonation of calcium hydroxide under high CO2-Ar pressure (90 bar) coupled with a complex selenocystine fragmentation under O2-poor and O2-rich conditions was carried out by using a semi-batch
system (sampling with time) in order to synthesize an elemental selenium (Se0)/calcite composite. Under O2-poor conditions (i.e. with purge stage), the composite was mainly characterized by spherical selenium nanoparticles
(<500nm) deposed on the calcite matrix. The carbonate matrix was constituted by nano- and micro rhombohedral crystals (<2µm) and micrometric agglomerates and/or aggregates (<5mµ). For this case, the spherical Se nanoparticles give
a stable red coloration to the composite. In contrast, under O2-rich conditions (i.e. without purge stage), the composite was characterized by hexagonal selenium microparticles (<25µm) dispersed in the calcite matrix. For this case, a gray coloration of composite was observed. In conclusion, the gas purge in the system and the selenocystine dose (mg/kgwater) play a crucial role on the selenocystine fragmentation mechanism during Ca(OH)2 suspension heating (at 90°C) and carbonation stages, this leading the precipitation/growth of elemental selenium with different morphologies and particles sizes. Consequently, it was noticed that the selenocystine fragmentation participates to the precipitation/growth of calcite with unusual morphologies. Finally, the results presented here demonstrate that Se0/calcite composite, with spherical or hexagonal morphologies for elemental selenium can be produced, this composite possibly with a high potential for medical (ex. dietary supplement) or industrial (ex. pigments) applications.