Efficient, Green, and Low-Cost Conversion of Bivalve-Shell Wastes to Value-Added Calcium Lactate

This work presents the efficient, green, and low-cost preparation of calcium lactate by using bivalve-shell wastes (cockle, mussel, and oyster shells) as raw materials. Three bivalve shells, a cockle, mussel, and oyster, were used separately as an alternative calcium-source material for the preparation of calcium lactate. The bivalve-shell waste was cleaned and milled, obtaining calcium carbonate (CaCO₃) powder, which reacted to the lactic acid, forming calcium lactate. The effects of different calcium sources (cockle, mussel, and oyster) and different lactic acid concentrations (6, 8, and 10 mol/L) on the physicochemical properties of the synthesized calcium lactates were then investigated. The results pointed out that the highest solubility of the product was observed when 6 mol/L lactic acid and cockle-shell derived CaCO₃ were employed for the calcium lactate preparation. The thermal decompositions of all calcium lactates occurred in three processes: dehydration, ethyl-lactate elimination, and decarbonization, respectively. The results, obtained from an infrared spectrometer, X-ray diffractometer, thermogravimetric analyzer, and scanning electron microscope, confirmed the formation of calcium lactate pentahydrate (Ca(CH₃CHOHCOO)₂·5H₂O). The diffractograms also indicated the presence of two enantiomers of Ca(CH₃CHOHCOO)₂·5H₂O, namely, of dl- and l-enantiomers, which depended on the lactic acid concentration used in the preparation process. The morphologies of calcium lactates show the firewood-like crystals in different microsizes, together with smaller irregular crystals. In summary, this work reports an effective process to prepare the valuable calcium lactates by using the cheap bivalve-shell-derived CaCO₃ as a renewable calcium source.