Sustainable conversion of oyster shell waste into high-purity calcium carbonate via CO2 mineralization

The accumulation of oyster shell waste is increasing annually, and the high costs associated with landfilling call for innovative solutions to repurpose this maricultural waste. This study proposes two methods for utilizing oyster shell waste as sources for CO2 mineralization: calcination and extraction. First, the calcination process consisting of two stages was investigated based on the characteristics of the oyster shell waste. This method yields high-purity CO2 gas through pretreatment, effectively removing organic compounds. Additionally, the calcination process significantly reduced the emissions of hazardous gases such as CO, NO, NO2, and SO2 by up to 72.97% during the first calcination stage. Second, a novel three-phase extraction process is introduced. This process achieved low pH conditions for leaching alkaline earth metals and high pH conditions for enhancing CO2 reactivity. It also does not generate acidic or basic wastewater, a common issue in chemical extraction. The extraction process demonstrated high Ca2 + extraction efficiency (99.07 %) and high-purity CaCO3 yield of 0.86ton CaCO3/ton of oyster shell waste. In CO2 mineralization, calcium ions derived from the calcination of oyster shell waste and extraction react with CO2, forming calcium carbonate. In addition, it was theoretically proved, based on dissociation constants, that CO32- is formed faster than HCO3- in the presence of OH- ions. The mechanism study revealed that alkaline wastewater acts as a pH buffer, enhancing CO2 reactivity and accelerating the conversion of CO2(g) to CO32-(aq), thereby promoting the growth and nucleation of CaCO3 by OH ions.