Carbonation of alkaline paper mill waste to reduce CO2 greenhouse gas emissions into the atmosphere

The global warming of Earth's near-surface, air and oceans in recent decades is a direct consequence of anthropogenic emission of greenhouse gases into the atmosphere such as CO2, CH4, N2O and CFCs. The CO2 emissions contribute approximately 60% to this climate change. This study investigates experimentally the aqueous carbonation mechanisms of an alkaline paper mill waste containing about 55 wt% portlandite (Ca(OH)2) as a possible mineralogical CO2 sequestration process. The overall carbonation reaction includes the following steps: (1) Ca release from portlandite dissolution, (2) CO2 dissolution in water and (3) CaCO3 precipitation. This CO2 sequestration mechanism was supported by geochemical modelling of final solutions using PHREEQC software, and observations by scanning electron microscope and X-ray diffraction of final reaction products. According to the experimental protocol, the system proposed would favour the total capture of approx. 218 kg of CO2 into stable calcite/ton of paper waste, independently of initial CO2 pressure. The final product from the carbonation process is a calcite (ca. 100 wt%)-water dispersion. Indeed, the total captured CO2 mineralized as calcite could be stored in degraded soils or even used for diverse industrial applications. This result demonstrates the possibility of using the alkaline liquid–solid waste for CO2 mitigation and reduction of greenhouse effect gases into the atmosphere.