Recycling of shell wastes into nanosized calcium carbonate powders with different phase compositions

• Nanosized CaCO3 powders were prepared from mollusk shells.
• Different phase compositions were achieved by adjusting the solution pH value.
• The intracrystalline macromolecules in the aragonite crystals were observable.
• There were different formation mechanisms for the aragonite and calcite phases.

Shell wastes represent a considerable quantity of by-products in the shellfish aquaculture. From the viewpoint of eco-friendly and economical disposal, it is highly desirable to convert these residues into high value-added products for industrial applications. In this study, shell wastes from Tapes japonica were used to prepare nanosized calcium carbonate powders by high energy ball-milling in the sodium hypochlorite solutions. The crystal phase of powders could be changed from flake-like aragonite to spindle calcite by simply increasing the alkalinity of milling solutions, revealing the mineralizing role of strong alkaline solution. The aragonite crystallites were 20–50 nm in size, and showed porous appearance due to the presence of intracrystalline organic macromolecules. The submicron-sized calcite particles were composed of close-packed nanocrystallites, presumably deriving from the organic-induced assembly of calcite phase. The formation of different phases was ascribed to the mechanically activated exfoliation of aragonite and the dissolution–recrystallization of calcite, during which the organic matrix was decomposed by the alkaline solutions. This mechanochemical method paves the way to the industrial production of calcium carbonate powders from shell wastes and thus the clean production of shellfish processing industry.